"Between the two prostrations, the Prophet would say: (Allahummaghfir li, warhamni, wajburni, wahdini, warzuqni). 'O Allah! Pardon me, have mercy on me, help me, guide me, and grant me sustenance.'"
Jami` at-Tirmidhi Book 2, Hadith 136
← Previous Hadith "We asked Ibn Abbas about squatting (sitting) on the heels. He said: 'It is the Sunnah.' We said: 'We think that it is difficult for a man.' He said: 'Rather, it is Sunnah of your Prophet.'"

Jami` at-Tirmidhi Book 2, Hadith 135 Next Hadith → (Another chain) which is similar.

Jami` at-Tirmidhi Book 2, Hadith 137