asalamu alikum is this from the obligations(wajibat) of salah(if so then what is the proof for that) (do we have to give sajda sahw if we miss it) Saying “Rabb ighfir li (Lord forgive me)” between the two prostrations

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

According to the view of the majority of the scholars the saying of the praying person ‘Rabi Aghfir li’ between the two prostrations is desirable and not an obligation of the obligations of the prayer because the Prophet did not order the person who did not know how to pray to say it. However, the Hanbali School of jurisprudence is of the view that it is a Waajib (obligation).

The Encyclopedia of Fiqh (Islamic jurisprudence) reads,

“In regard to the sitting between the two prostrations, it is a Sunnah to seek forgiveness [i.e. to say ‘Rabi Aghfir li’] according to the Hanafi, Maaliki and Shaafi’i Schools of jurisprudence, and it is also a narration from Ahmad. The evidence for this is that Huthayfah prayed with the Prophet and he [i.e. the Prophet] used to say between the two prostrations: ''‘Rabi Aghfir li, Rabi Aghfir li’. Nonetheless, it is not an obligation to seek forgiveness [i.e. to say ‘Rabi Aghfir li’] because the Prophet did not teach the person who did not know how to pray to say it. However, the known view of the Hanbali School is that it is a Waajib (obligation), and this is the view of Is-Haaq and Daawood.”

The evidence which the Hanbali School provided for saying that it is an obligation is that this was confirmed from the Prophet as he said: "Pray as you see me praying.” According to them, if one deliberately leaves it, his prayer becomes void, and if one leaves it out of forgetfulness, he must prostrate for forgetfulness.

Al-Mughni, authored by Ibn Qudaamah reads, “If one forgets something among the obligatory mention [Ath-kaar], like the Tasbeeh in the bowing down position [Rukoo’] and in the prostration [Sujood], and saying ‘Rabi Aghfir li’ between the two prostrations, and the saying of ‘Rabana walaka Al-hamd’, then he should not go back to them once he has moved from the position where he should say it; rather, he should continue [his prayer] and prostrate for forgetfulness because of leaving it, by analogy to leaving the Tashahhud.”

In our view, the preponderant opinion is the view of the majority of the scholars as the Prophet mentioned to the person who did not know how to pray what he should do and say so that his prayer be valid and he told him: "The prayer of one of you is not complete until he……” and he did not mention to him seeking forgiveness [i.e. saying ‘Rabi Aghfir li’] between the two prostrations, and he did not mention to him the Tasbeeh in the bowing position and in the prostration because had this been an obligation [Waajib], he would have clarified it to him. Rather, it is a recommended Sunnah (Prophetic tradition) as it was confirmed that the Prophet did it. 

Allaah Knows best.