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