Is Saying `Rabb Ighfir Li’ Mandatory?
Question

 

According to Hanafi madhab, is it wajib or sunnah to say a Dua in Jalsa? Like saying "rabbighfirli" 3 times or any other dua.
I just recently found out about this and am confused on whether it is sunnah or wajib.
If it is wajib, will all my past salah count?
Also, is it wajib or sunnah to say Takbir when changing postures in salah (i.e. qiyam to Ruku, Ruku to sujood etc...)
Praise be to Allah.Is saying `Rabb Ighfir Li’ mandatory?
The scholars differed concerning the ruling on the Takbirs of movement in the prayer and on saying between the two prostrations, “Rabb Ighfir Li (Lord forgive me).” There are two opinions:

The first opinion, which is the view of the majority of Hanafis, Malikis and Shafi`is, is that the Takbirs of movement and saying “Rabb ighfir li (Lord forgive me)” between the two prostrations come under the heading of the Sunnahs of prayer and are not obligatory parts of the prayer.
The second opinion is that they are obligatory parts of the prayer. This is the view of the Hanbalis.

Ibn Qudamah (may Allah have mercy on him) said in Al-Mughni (1/298): The well-known view narrated from Ahmad is that the Takbir said when going down and coming up, the Tasbihs of bowing and prostration, saying “Sami`a Allahu liman hamidah, Rabbana wa laka Al-hamd (Allah hears those who praise Him, our Lord to You be praise)”, saying “Rabb ighfir li (Lord forgive me)”, saying “Rabb ighfir li (Lord forgive me)” between the two prostrations, and the first Tashahhud, are all obligatory. It is also the view of Is-haq and Dawud.
And it was narrated from Ahmad that it is not obligatory, which is the view of most of the jurists, because the Prophet (blessings and peace of Allah be upon him) did not teach it to the one who prayed improperly, and it is not permissible to delay explaining something at the time when it becomes necessary. 
Moreover, Ibn Qudamah quoted a number of things to indicate that it is obligatory:

The Prophet (blessings and peace of Allah be upon him) enjoined it, and his instruction indicates that something is obligatory.
He did it and he said: “Pray as you have seen me praying.”
Abu Dawud (856) narrated from `Ali ibn Yahya ibn Khallad from his paternal uncle that the Prophet (blessings and peace of Allah be upon him) said: “The prayer of one of the people is not complete unless he does wudu’ … Then he says: Allahu Akbar, then bows.” Classed as authentic by Al-Albani in Sahih Abu Dawud.
Because the points at which these Adhkars are recited are pillars or essential parts of the prayer, dhikr at these points is obligatory, just as there are words to be recited when standing.

With regard to the Hadith about the one who prayed improperly, it is mentioned in the Hadith that we narrated that he (the Prophet, blessings and peace of Allah be upon him) taught him that. It is something additional that must be accepted.  The Prophet (blessings and peace of Allah be upon him) did not teach you all the obligatory parts of the prayer, based on the fact that he did not teach the Tashahhud or the Salam. So it is to be understood as meaning that he limited it to teaching him only that which he had seen him doing improperly.”
It says in Al-Mawsu`ah Al-Fiqhiyyah (4/40):
When sitting between the two prostrations, it is Sunnah to pray for forgiveness, according to the Hanafis, Malikis and Shafa`is, and it is one opinion that was narrated from Ahmad. The basis that is to be found in the report narrated by Hudhayfah, according to which he prayed with the Prophet (blessings and peace of Allah be upon him) and he used to say between the two prostrations: “Rabb ighfir li, Rabb ighfir li (Lord forgive me, Lord forgive me).”
However, asking for forgiveness here is not obligatory, because the Prophet (blessings and peace of Allah be upon him) did not teach it to the one who prayed improperly. The well-known view of the Hanbalis is that it is obligatory. This is also the opinion of Is-haq and Dawud, and saying it one time is sufficient.” 
This is a matter concerning which there is a difference of opinion among the scholars. The most that can be said is that the view of the Hanbalis, that the Takbirs of movement are obligatory, is more likely to be correct, because of the evidence mentioned above, and because the Prophet (blessings and peace of Allah be upon him) said: “The imam has been appointed to be followed, so when he says Takbir, then say Takbir…” (Narrated by Muslim, no. 411)
With regard to the Du`a between the two prostrations, the opinion of the majority, which is that it is recommended, is more likely to be correct, because there is no evidence to clearly indicate that it is obligatory.
What to say between Sujud
It is better for the worshipper to recite Du`as that have been narrated in the texts between the two prostrations. With regard to adding to the Du`as that have been narrated, or saying a Du`a that was not narrated , what appears to be the case is that it is permissible. 
And Allah knows best.