What to Say Between Sujud
Question

Is it Wajib or Sunnah to say a Du`a in Jalsa? Like saying "rabbighfirli" 3 times or any other Du`a. 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?
Praise be to Allah.What to say between Sujud
It is better for the worshipper to say Du`a between the two Sujud (prostrations) in the words narrated from the Prophet (blessings and peace of Allah be upon him), as it was narrated by An-Nasa’i (1145) from Hudhayfah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) used to say between the two prostrations: “Rabb ighfir li, Rabb ighfir li (Lord forgive me, Lord forgive me).” (Classed as authentic by Al-Albani in Sahih Sunan An-Nasa’i)
At-Tirmidhi (262) narrated from Ibn `Abbas (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) used to say between the two prostrations: “Allahumma ighfir li wa’rhamni wa’jburni wa’hdini wa’rzuqni (O Allah, forgive me, have mercy on me, console me, guide me and grant me provision).” (Classed as authentic by Al-Albani in Sahih Sunan At-Tirmidhi)
Can you add anything to the Du`a between  Sujud?
With regard to adding to the Du`a that was narrated from the Prophet (blessings and peace of Allah be upon him) or using words other than those that were narrated, what appears to be the case is that it is permissible, because this is a posture (of the prayer) in which Du`a is allowed. However it is preferable – as stated above – to limit it to what was narrated from the Prophet (blessings and peace of Allah be upon him), and to say Du`a in words other than those which were narrated when prostrating or before saying the Salam at the end of the prayer. Muslim (479) narrated from Ibn ‘Abbas (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “and as for prostrating, strive in Du`a, for it is deserving of a response (from your Lord).”
Shaykh Ibn Baz (may Allah have mercy on him) said: 
“Then he should rise from prostration and say, “Rabb ighfir li, Rabb ighfir li, Rabb ighfir li (Lord forgive me, Lord forgive me, Lord forgive me),” as the Messenger (blessings and peace of Allah be upon him) used to do. And it is recommended to say along with that: “Allahumma aghfir li wa’rhamni wa’hdini wa’jburni wa’rzuqni wa ‘afini (O Allah, forgive me, have mercy on me, guide me, console me, grant me provision and grant me well being),” because it is proven that the Prophet (blessings and peace of Allah be upon him) used to say that. And if he wants to add to that, there is nothing wrong with it, such as saying: O Allah, forgive me and my parents; O Allah, admit me to Paradise and save me from the Fire; O Allah, rectify my heart and my deeds, and so on. But he should offer a lot of Du`a for forgiveness between the two prostrations, as was narrated from the Prophet (blessings and peace of Allah be upon him).” (Majmu` Fatawa Ash-Shaykh Ibn Baz, 11/36)
Shaykh Muhammad ibn Muhammad Al-Mukhtar Ash-Shanqiti (may Allah preserve him) said: 
“The Prophet (blessings and peace of Allah be upon him) used to ask for well-being between the two prostrations, because this is the place of Du`a. The scholars had two opinions concerning that. Some of them said: It is a place for Du`a, so it is prescribed for a person to offer whatever Du`a he likes concerning his affairs in this world and in the Hereafter. Others said: He should limit it to what has been narrated (from the Prophet (blessings and peace of Allah be upon him)), and this view is stronger. But if he has not memorised it, such as if he is one of the ordinary people, there is nothing wrong with him saying whatever Du`a he can.
The reason why he may say whatever Du`a he can is that the Sunnah is two things: the first is saying Du`a and the second is the specific wording. If he is not able to say the specific wording, then the Sunnah is to ask of Allah. This is the view of the majority, as more than one scholar attributed it to them. So if he is not able to memorise the wording that was narrated, or he does not know it, and he says whatever Du`a he is able to, such as offering Du`a for himself and his parents, there is nothing wrong with that. But it is preferable and better to say the Du`a that was narrated from the Prophet (blessings and peace of Allah be upon him).” (Sharh Az-Zad)
For more details, please see these answers: 7886 , 186625 , and 46997 .     
And Allah knows best.