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