Dought and clear - The rights of one Muslim over another include those that are obligatoryand those that are mustahabb.

We know the hadeeth of the Messenger (blessings and peace of Allah be
upon him) about the rights of one Muslim over another. Myquestion is:
Will we be sinning if we do not fulfil one of these rights towards our
Muslim brother? i.e., will we incur a burden of sin thereby?.
Praise be to Allaah.
The rights that one Muslim has over another are many, some of whichare
individual obligations that are required of each person,and if he
fails to do themhe is sinning. Others are communal obligations; ifsome
people do them, the burden of sin is waived for the rest. And some are
mustahabb (encouraged) but not obligatory, so the Muslimis not sinning
if he does not do them.
Al-Bukhaari (1240) and Muslim (2162) narrated that Abu Hurayrah (may
Allah be pleased with him) said: I heard the Messenger of Allah
(blessings and peace of Allah be upon him) say: "The rights of one
Muslim over another are five: returning the greeting of salaam,
visiting the sick, attending funerals, accepting invitations, and
sayingyarhamuk Allah(may Allah have mercy on you) to one who sneezes."
And Muslim (2162) narrated from Abu Hurayrah (may Allah be pleased
with him) that the Messenger of Allah (blessings and peace of Allah be
upon him) said: "The rights of one Muslim over another are six." It
was said: What are they, O Messenger of Allaah? He said: "If you meet
him, greet him with salaam; if he invitesyou, accept the invitation;
if he asks for advice, give him sincere advice; if he sneezes
andpraises Allaah, sayYarhamuk Allaah(may Allaah have mercy on you);
if he falls sick, visit him; and if he dies, attend his funeral."
Ash-Shawkaani (may Allah have mercy on him) said: What is meantby the
words "The rightsof the Muslim" is that they should not be omitted and
doing them is either obligatory, or recommended to such an extent that
it is very similar to being obligatory and should not be omitted. The
word "right" (haqq) maybe used in the sense of obligatory, as was
mentioned by Ibn al-A'raabi.
End quote fromNayl al-Awtaar, 4/21
1.
Returning the greeting of salaam is obligatory ifthe greeting is given
to one person. If it is given to a group, then it is obligatory upon
the group (fard kifaayahor communal obligation; if one of the group
returnsthe greeting, the obligation as been met). With regard to
initiatingthe greeting, the basic principle is that it is Sunnah. It
says inal-Mawsoo'ah al-Fiqhiyyah(11/314):
Initiating the greeting is Sunnah mu'akkadah (a confirmed Sunnah)
because the Prophet (blessings and peace of Allah be upon him) said:
"Spread the greeting of salaam amongst yourselves." It is obligatory
to return the greeting if it is given to one person. If the greeting
is given to a group, then in their caseresponding is a fard kifaayah
(communal obligation); if one of them responds the sin is waived from
the others, but if they all respond, they have all done what is
required, Whether they respond all together or one after another. If
they all refrain from responding,then they are all sinning because of
the report which says: "The rights of one Muslim over another are
five: returning (the greeting) of salaam…"
End quote.
2.
Visiting the sick it is a communal obligation. Shaykh Ibn 'Uthaymeen said:
Visiting the sick is a fard kifaayah (communal obligation).
Majmoo' Fataawa wa Rasaa'il Ibn 'Uthaymeen,13/1085
See also the answer to question no. 71968
3.
Attending funerals is also a fard kifaayah (communal obligation). See
the answer to question no. 67576
4.
With regard to acceptinginvitations, if the invitation is to a wedding
feast, then the majority of scholars are of the view that it is
obligatory to accept unless there is a legitimate shar'i reason not to
do so. If it is for something other than a wedding feast, the majority
are of the view that it is mustahabb. But there are conditions for
accepting invitations in general terms. For details of that please
seethe answer to question no. 22006
5.
With regard to sayingYarhamuk Allah(May Allah have mercy on you)to one
who sneezes, there is a difference of opinion concerning the ruling.
It says inal-Mawsoo'ah al-Fiqhiyyah, 4/22:
This sayingYarhamuk Allah(May Allah have mercy on you) is Sunnah
according to the Shaafa'is.
According to the Hanbalis and the Hanafis, it is obligatory.
The Maalikis said – and itis an opinion among the Hanbalis – that it
is a communal obligation. It was narrated from al-Bayaan that the
stronger view is that it isan individual obligation (fard 'ayn),
because of the hadeeth: "It is the duty of every Muslim who hears him
(the one who sneezes) to say: Yarhamuk Allah (may Allah have mercy on
you). End quote
The more correct opinion is that it is obligatory on the one who hears
the sneezer praise Allah (by saying "Al-hamdu Lillah"), because of the
report narrated by al-Bukhaari (6223) from Abu Hurayrah (may Allah be
pleased with him) from the Prophet (blessings and peace of Allah be
upon him) who said: "Allaah likes the act of sneezing and dislikes
theact of yawning, so if any one of you sneezes and praises Allaah
(says 'al-hamdu Lillaah'), it is aduty on every Muslim who hears him
to say to him, 'Yarhamuk Allaah (may Allaah have mercy on you).'"
Ibn al-Qayyim (may Allah have mercy on him) said:We have quoted above
the hadeeth of Abu Hurayrah, in which it says: "if any one of you
sneezes and praises Allaah (says 'al-hamdu Lillaah'), it is a duty
(haqq) on every Muslim who hears him to say to him, 'Yarhamuk
Allaah(may Allaah have mercy on you).". al-Tirmidhi included the
hadeeth of Anas under the heading: Chapter on what was narrated about
it being obligatory to sayYarhamuk Allah(may Allah have mercy on
you)when one who sneezes saysAl-hamdu Lillah(Praise be to Allah). This
indicates that it is obligatory in his view, and this is the correct
view, because of the hadeeths that clearly indicate that it is
obligatory and there was nothing to contradict that, and Allah knows
best.
One of them is the hadeeth of Abu Hurayrah mentioned above, and
another is his other hadeeth, "There are five (rights) that the Muslim
has over his brother," which is also mentioned above. Another is the
hadeeth of Saalim ibn 'Ubayd, in which it says: "Let those who are
with him say to him:Yarhamuk Allah(may Allah have mercy on you)." And
another is the report narrated by at-Tirmidhi from 'Ali who said: The
Messengerof Allah (blessings and peace of Allah be upon him) said:
"The Muslim has six (rights) over his fellow-Muslim: he shouldgreet
him with salaam when he meets him; he should respond when heinvites
him; he should sayYarhamuk Allah(may Allah have mercy on you)if he
sneezes; he should visit him if he falls sick; he should attend his
funeral if he dies; and heshould love for him whathe loves for
himself." He (at-Tirmidhi) said: This is a hasan hadeeth. It was also
narrated via another isnaad from the Prophet (blessings and peace of
Allah be upon him). Some of the scholars spoke negatively about
al-Haarith al-A'war (one of the narrators). In the same chapter it is
also narrated from Abu Hurayrah, Abu Ayyoob, al-Bara' and Abu Mas'ood.
And another of these hadeeths is that which was narrated by
at-Tirmidhi from Abu Ayyoob, according to which the Messenger of Allah
(blessings and peace of Allah be upon him) said: "If one of you
sneezes, let him sayAl-hamdu Lillah(praise be to Allah); and let him
say'ala kulli haal(in all circumstances). And let the one who responds
tohim say:Yarhamuk Allah(may Allah have mercy on you). And let him
say:Yahdikum Allahu wa yuslih baalakum(May Allah guide you and set
your affairs straight).
There are four kinds of proof in the hadeeth quoted above ("if any one
of you sneezes and praises Allaah (says 'al-hamdu Lillaah'), it is
aduty (haqq) on every Muslim who hears him tosay to him, 'Yarhamuk
Allaah (may Allaah have mercy on you)"):
(i)there is a clear statement that it is obligatory to sayYarhamuk
Allah(may Allah have mercy on you), which cannot be interpreted in any
other way;
(ii)it is made obligatory by use of the wordhaqq(translated above as duty);
(iii)it is made obligatory by use of the word'ala(on). This word
clearly means that it is obligatory
(iv)it is enjoined. There can be no doubt that there are many duties
that are proven to be obligatory on the basis of less evidence than
this. And Allah knows best.
End quote fromHaashiyat Ibn al-Qayyim 'ala Sunan Abi Dawood, 13/259
He also said: The apparent meaning of the hadeeth mentioned above is
that sayingYarhamuk Allahis an individual obligation upon everyone who
hears the one who sneezed sayAl-hamdu Lillah; it is not acceptable for
just one of them to say it. This is one of the two scholarly opinions,
which was favoured by the Maalikis Ibn Abi Zayd and Abu Bakr ibn
al-'Arabi, and it cannot be otherwise.
End quote fromZaad al-Ma'aad, 2/437
6.
With regard to giving him advice if he asks for it, it is most likely
that offering advice is a communal obligation.
Ibn Muflih (may Allah have mercy on him) said:
The apparent meaning of the words of Ahmad and our companions is that
it is obligatory to offer advice to the Muslim, even if he does not
ask for it, as is the apparent meaning of the reports.
End quote fromal-Adaabash-Shar'iyyahby Ibn Muflih, 1/307
Al-Mullah 'Ali al-Qaari (may Allah have mercy on him) said:
"If he asks you for advice" means if he asks you for advice, then
giveit to him; it is obligatory.It is also obligatory to give advice
even if he did not ask for it.
End quote fromMirqaat al-Mafaateeh, 5/213
al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
It is clear that what is meant by "duty" (haqq) here is that it is
obligatory. This is different from the wordsof Ibn Battaal who said
that what is meant is theduty of respect and companionship. It
seemsthat what is meant here is that it is a communal obligation.
End quote fromFath al-Baari, 3/113
And Allah knows best.