Assalamu alaykum wa rahmatullaahi wa barakaatuhu. I have a question regarding the prayer. After the Takbeer (saying Allaahu Akbar) from the second prostration in an odd Rak’ah (unit of prayer), can I sit and supplicate ''Rabi Ighfir Li'' (My Lord, forgive me) twice before standing for next Rak’ah, or is that considered an addition to the prayer? I did it one time when I was praying by myself, and I did it another time when I was praying behind the imam. I know that I can sit for a short time after the second prostration before standing for the next Rak’ah; this sitting is called Jalsat-ul-Istiraahah, but I learned lately that this should be done without supplication or Thikr (expression of remembrance of Allaah). Were my two prayers correct, or do I have to repeat them? And If I repeated this action unintentionally in the future, should I do Sujood As-Sahw (prostration of forgetfulness)?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

The scholars differed in opinion about the Jilsat-ul-Istiraahah, which is a brief pause; sitting for a short while before standing up to do the following Rak’ah after the first or third Rak’ah.

Some scholars said that it is permissible at all times, and some stated that it is not at all permissible. There is a third view that it is permissible when needed. These three views are mentioned by Shaykh Ibn ‘Uthaymeen in his book Ash-Sharh Al-Mumti’ ‘ala Zaad Al-Mustaqni’:

“The first view: the worshiper does not sit, as mentioned by the author; it is not a Sunnah to sit at all. This is the adopted view (of the Hanbali Mathhab).

The second view: the worshiper sits in all cases, whether or not he needs to sit down for this pause; he sits as an act of worship to Allah, Almighty. This is the view of most scholars of Hadeeth, and these are two contradictory opinions [i.e. the first and second views.]

The third view is moderate between the two: in some cases it conforms to the first view, and in some other cases, it conforms to the second view. They said, 'if a person needs to sit down; i.e. he cannot get up without sitting, then he sits as an act of worship, and if he can stand up right away, then he does not sit down.' This is the view chosen by Ibn Qudaamah and Ibn Al-Qayyim. Each of these three different views has supporting evidence…”

Nonetheless, Jilsat-ul-Istiraahah is short, and there is no supplication to be said in it; Shaykh Ibn ‘Uthaymeen said, “There is no supplication for Jilsat-ul-Istiraahah.”

Regarding the supplication that you said in Jilsat-ul-Istiraahah, it is like adding a permissible mention of Allah, but not at its proper place, and this does not invalidate the prayer, even if you said it deliberately.

Ibn Qudaamah said:

“The second kind: what does not invalidate the prayer if it is said deliberately: this is of two types: one is to say a permissible Thikr in prayer, but not at its proper place, such as reading the Quran while bowing or prostrating, or saying the Tashahhud while in the standing position, or saying the salutations on the Prophet in the first Tashahhud, and reciting a Soorah in the last two Rak’ahs in prayers of four units [Thuhr, Asr, and Isha], or in the last Rak’ah in the Maghrib prayer, and the like, if he does it out of forgetfulness; is it permissible for him to prostrate for forgetfulness? There are two narrations on this, one of which is that he is not permitted to prostrate because the prayer is not invalidated if this is done deliberately, so it is not permitted to prostrate if one has done it out of forgetfulness.” 

Based on the above, your prayer is correct. It does not become invalidated because of saying a supplication in Jilsat-ul-Istiraahah, but you should not say any supplication in this position in the future because this Jilsat-ul-Istiraahah is not a position for saying a supplication or mention of Allah.

Allah knows best.