How can he advise someone who is falling short with regard to prayer?

How can I call someone to pray who, even though he knows that it is
obligatory, does not do it sometimes? I wouldlike you to tell me some
simple phrases that haveto do with this question, and some phrases
that will warn of punishmentand encourage seeking reward.
Praise be to Allaah.
You can remind this person and advise him by explaining the ruling on
not praying, and on not praying in congregation in the mosque, and
explain how the early generations of Muslims were with regard to
prayer.
The ruling on not praying:
In a number of questionswe have explained that the one who does not
pray is a kaafir who is beyond the pale of Islam; no good deeds will
be accepted from him on the Day of Resurrection; and his marriage
contract with his wife must be annulled if she does pray. This one who
does not pray should also understand that any meat slaughtered by
himcannot be eaten; and (if he dies) he cannot be washed, have the
funeral prayer offered for him, or be buried in the Muslim graveyard.
It was narrated from Samurah ibn Jundub (may Allah be pleased with
him) that the Messenger of Allah (blessings and peace of Allah be upon
him) mentioned a dream that he had seen and said: "Last night two
people came to me and said, 'Let's go,' so I went with them. We came
to a manwho was lying down, with another man standing over him,
holding a big rock. He threw the rock at the man's head, smashing it.
The rock rolled away, and the one who had thrown it followed it, and
picked it up. By the time he came back to the man, his head had been
restored to its former state. Then he (the one who had thrown the
rock) did thesame as he had done before. I said to (my two
companions), 'Subhaan Allaah! What is this?' They said to me: Move
on…"
At the end of the hadeeth, he (blessings and peace of Allah be upon
him) said: "They said to me: 'The first manwhom you saw with his head
being smashed with the rock, is the manwho learned the Qur'aanthen
abandoned it, and he went to sleep, neglecting the obligatory
prayers.'"
Narrated by al-Bukhaari,6640
Muhammad ibn Nasr al-Marwazi said:
I heard Ishaaq – i.e., ibn Raahawayh – say: It was narrated in a
saheeh report from the Prophet (blessings and peace of Allah be upon
him) that the one who does not pray is a kaafir. It was the opinion of
the scholars from the time ofthe Prophet (blessings and peace of Allah
be upon him) that the one who deliberately does not pray, without any
excuse, until the time forthe prayer ends, is a kaafir. The time for
the prayer ending means delaying Zuhr until sunset and Maghrib
untilsunrise.
Ta'zeem Qadr as-Salaah, 2/929
For the ruling on the onewho does not pray, please see the answers to
questions no. 7864 , 5208 and 2182
With regard to praying in congregation in the mosque:
Just as the prayer is what differentiates between the Muslim andthe
kaafir, praying in congregation is the sign that differentiates
between the believer and the hypocrite. The Sahaabah (may Allah be
pleased with them) regarded the one who did not attend prayer in
congregation as a hypocrite whose hypocrisy was known.
Shaykh Saalih al-Fawzaan (may Allah preserve him) was asked:
I am a young man, not married, and I pray but not continually, i.e., I
do not always offer the prayer on time, and sometimes I miss the
prayers for a whole day, or I do them all together.What is the Islamic
ruling on that?
He replied:
The Muslim is required to offer the prayers regularly on time, with
the congregation of Muslims. It is not permissible to delay the prayer
until its time is over. Allah, may He be exalted, says (interpretation
of the meaning): "Verily, the prayer is enjoined on the believers at
fixed hours" [an-Nisa' 4:103], i.e., it is enjoined at specific times,
at which it should be offered. According to a report: "Allah has deeds
at nightthat He does not accept during the day, and deeds during the
day that He does not accept at night." Delaying the prayer until the
time for it is over is neglecting it. Allah, may He be exalted,says
(interpretation of the meaning): "Then, there has succeeded them a
posterity who have given up As-Salat (the prayers) (i.e. made their
Salat (prayers) to be lost, either by not offering them or by not
offering them perfectly or by not offering them in their proper fixed
times, etc.) and have followed lusts. So they will be thrown in Hell,
except those who repent…" [Maryam 19:59,60].
Putting two prayers together is not permissible, unless thereis a
legitimate shar'i excuse at the time of one of them, such as putting
Zuhr together with 'Asr, or Maghrib together with 'Isha'. But putting
the prayers of anentire day together is not permissible and prayers
are not valid if done in this manner.
Al-Muntaqa min Fataawaal-Fawzaan, 5/56, 57
You will find the evidence for prayer in congregation being obligatory
in the answers to questions no. 120 and 8918 .
With regard to how the early generations of Muslims were with regard
to praying in congregation in the mosque:
They regarded it as something of the utmostimportance, to which they
paid a great deal ofattention and were keento offer the prayer in
congregation in the mosque even if they hadexcuses not to do so.
Wakee' ibn al-Jarraah said, narrating from al-A'mash Sulaymaan
ibnMahraan: al-A'mash was nearly seventy years old but he never missed
the first takbeer.
Muhammad ibn al-Mubaarak as-Soori said: If Sa'eed ibn 'Abd al-'Azeez
al-Tanookhi missed the prayer in congregation, he would weep.
It was narrated from Muhammad ibn Khafeef that he had a pain in his
side that, when it affected him, prevented him from moving. But when
the call to prayer was given, he would be carried on a man's back. It
was said to him: Why don't you take it easy onyourself? He said: If
you hear (the words of the adhaan), Hayya 'ala as-salaah (come to
prayer) and you do not see me in the row (of worshippers), then look
for me in the graveyard.
'Aamir ibn 'Abdillah ibn az-Zubayr heard the mu'adhdhin when he was
dying and he said: Hold my hand (and take me to the prayer). It was
said: You are sick. He said: I can hear the callerof Allah; should I
not respond to him? So they took him by the hand and he joined the
imam in praying Maghrib; he prayed one rak'ah then he died.