It wasnarrated from Hudhaifah that the Prophet (ﷺ) used to saybetweenthe two prostrations:
"Rabbighfir li, Rabbighfir li (O Lordforgiveme, O Lord forgive me)." (ID)
English reference (ID): Vol. 1, Book 5, Hadith 897 (ID)
Arabic reference (ID): Book 5, Hadith 947 (ID)