رَبِّ اغْفِرْ لِي، رَبِّ اغْفِرْ لِي

Rabbighfir lee, Rabbighfir lee

O My Lord, forgive me. My Lord, forgive me. Huzaifa (RA) said that the Prophet (ﷺ) used to recite the above mentioned dua while sitting in the middle of two prostrations.

Reference: Sahih (Albani). Abu Dawud: 874