1] Reasons for doing the prostration of forgetfulness

1]
When is it prescribed for aworshipper to do the prostration of forgetfulness?.
Praise be to Allaah.
By the mercy of Allaah to His slaves, as one of the beauties of this
perfect religion, Allaah has prescribed that His slaves may make up
for shortcomings and mistakes that they make in their worship and
cannot avoid completely, by performing supererogatory (naafil) acts of
worship, praying for forgiveness and so on.
One of the things that Allaah has prescribed for His slaves to make up
for shortcomings that may occur in their prayer is theprostration of
forgetfulness, but it is only prescribed to make up for certain
things; it does not make up for everything nor is it prescribed for
everything.
Shaykh Ibn 'Uthaymeen (may Allaah have mercy on him) was asked about
the reasons for doing the prostration of forgetfulness, and he replied
as follows:
The prostration of forgetfulness in prayer is generally prescribed for
three reasons:
1- Doing something extra
2- Omitting something
3- Being uncertain
Doing something extra: for example, doing an extra bow (rukoo'),
prostration (sujood), standing or sitting.
Omitting something: such as omitting an essential part of the prayer,
or omitting one of the obligatory parts of the prayer.
Being uncertain: such as when a person is not sure how many rak'ahs he
has prayed, whether it was three or four, for example.
In the case of doing something extra, if a person adds something tohis
prayer – bowing, prostrating, standing or sitting – deliberately,
thenhis prayer becomes invalid, because when he added it, that means
that he did not do the prayer in the manner ordained by Allaah and His
Messenger (peace and blessings of Allaah be upon him). The Prophet
(peace and blessings of Allaah be upon him) said: "Whoever does an
action that is not part of this matter of ours (i.e., Islam),will have
it rejected." Narrated by Muslim, 1718.
But if he did that extra thing by mistake, then hisprayer is not
invalidated, but he should do the prostration of forgetfulness after
saying the salaam. The evidence for that is the hadeeth of Abu
Hurayrah (may Allaahbe pleased with him) whosaid that when the Prophet
(peace and blessings of Allaah be upon him) said the salaam after two
rak'ahs in one of the two afternoon prayers, either Zuhr or 'Asr, and
they told him about that, he (peace and blessings of Allaah be upon
him) did the rest of the prayer, then he said the salaam, then he
prostrated twice (the prostration of forgetfulness) after sayingthe
salaam. Narrated by al-Bukhaari, 482; Muslim, 573.
And Ibn Mas'ood (may Allaah be pleased with him) narrated that the
Prophet (peace and blessings of Allaah be upon him) led them in
praying Zuhr and he prayed five rak'ahs. When he had finished it was
said to him: "Has something been added to the prayer?"
He said: "Why are you asking that?"
They said: "You prayed five (rak'ahs)."
So he turned towards the qiblah and prostrated twice. Narrated by
al-Bukhaari, 4040; Muslim,572.
In the case of omitting something, if a person omits one of the
essential parts of the prayer, one ofthe following two scenarios must
apply:
Either he remembers it before he reaches the same point in the
following rak'ah, so he has to go back and do that essential part of
the prayer and whatever came after it,
Or he does not remember it until he reaches the same point in the
following rak'ah, in which case the following rak'ah takes the place
of the one in which he omitted that essential part, and he should make
up the invalid rak'ah.
In either of these two cases, he should do the prostration of
forgetfulness after saying the salaam.
For example: a man stoodup after doing the first prostration of the
first rak'ah and did not sit or do the second prostration.When he
started to recite Qur'aan he remembers that he had not done the second
prostration or sat between the two prostrations. In that case he
should go back and sit as between the two prostrations, then
prostrate, then stand up and do whatever is left of his prayer, and do
the prostration of forgetfulness after saying the salaam.
An example of one who did not remember until after he reached the
samepoint in the following rak'ah is a man who stoodup following the
first prostration in the first rak'ah and did not do the second
prostration or sit between the two prostrations, but he did not
remember that until he sat between the two prostrations in the
secondrak'ah. In this case he should make the second rak'ah the first
rak'ah, andadd another rak'ah to his prayer, then say the salaam then
do the prostration of forgetfulness. :->