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