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