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