Ruling on the takbeers of movement (in the prayer) and the du‘aa’ between the two prostrations

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 takbeer when changing posturesin
salah (i.e. qiyam to ruku, ruku to sujood etc...)
Praise be to Allah.
Firstly:
The scholars differed concerning the ruling onthe takbeers 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,
Maalikis and Shaafa'is, is that the takbeers of movement and saying
"Rabb ighfir li (Lord forgive me)" between the two prostrations come
under the headingof the Sunnahs of prayerand 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 Qudaamah (may Allah have mercy on him) said in al-Mughni (1/298):
The well known view narrated from Ahmad is that the takbeer said when
going down and comingup, the tasbeehs of bowing and prostration,saying
"Sami' Allahu liman hamidah, Rabbanawa 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 Ishaaq and Dawood.
And it was narrated fromAhmad that it is not obligatory, which is the
view of most of the fuqaha', 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 Qudaamah quoted a
number of things to indicate that it is obligatory:
1. The Prophet (blessings and peace of Allah be upon him) enjoined
it, and his instruction indicates thatsomething is obligatory.
2. He did it and he said: "Pray as you have seen me praying."
3. Abu Dawood (856) narrated from 'Ali ibn Yahya ibn Khallaad 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 wudoo' …Then he says: Allahu akbar, then bows." Classed
as saheeh by al-Albaani in Saheeh Abi Dawood.
4. Because the points at which these adhkaar 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 hadeeth about the one who prayed improperly, it is
mentioned in the hadeeth 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 [[cxn??] The Prophet
(blessings and peace of Allah be upon him) did not teach you all the
obligatory parts of the prayer, based on the factthat he did not
teaching the tashahhud or the salaam. So it is to be understood as
meaning that he limited it to teaching him only that which he had seen
him doing improperly. End quote.
It says in al-Mawsoo 'ah al-Fiqhiyyah (4/40):
When sitting between the two prostrations, it is Sunnah to pray for
forgiveness, according to the Hanafis, Maalikis and Shaafa '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 Ishaaq and Dawood,
and aqallahu one time [??]. End quote.
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 takbeers of movement are obligatory, is more
likelyto be correct, because ofthe evidence mentioned above, and
because the Prophet (blessings and peace of Allah be upon him) said:
"The imam hasbeen appointed to be followed, so when he says takbeer,
then say takbeer…" Narrated by Muslim, no. 411.
With regard to the du 'aa' between the two prostrations, the opinionof
the majority, which is that it is mustahabb, is more likely to be
correct,because there is no evidence to clearly indicate that it is
obligatory.
For more information please see the answers to questions no. 130981 and 134965
Secondly:
it is better for the worshipper to recite du 'aa's that have been
narrated in the texts between the two prostrations. With regard to
adding to the du 'aa's that have been narrated, or saying a du 'aa'
that was not narrated, what appears to be the case is that it
ispermissible.