Answered by: Maulana Muqtaur Rahman
Between the two sujood’s if one does not read ‘Rabbigfirli’ will his Salah be not valid
In the name of Allah, the Most Gracious, the Most Merciful
The Sajdah/Sujood is one of the compulsory acts of Salah without which a person’s Salah will be categorized as invalid. The Sajdah is also one of the positions which expresses ultimate submission that a person can possibly offer.
In regard to your query, reciting ’Ribbighrif-Lee’ or any other duas in between the two Sujoods is not from the requirements for the Sujood to be valid. Therefore, if one does not recite any duas in between the two Sujoods, the Salah will be valid.
Only Allah knows best
Written by Maulana Muqtaur Rahman
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 Raddul-Muhtar, v.1 pg.447, Darul-Fikr
Al-Lubab, v.1 pg.71 ,DKI