In sajdah, one should recite سُبْحَانَ رَبِّيَ الْأَعْلَى thrice.
Reciting the following dua in sajdah in one’s nafl salaah is also established in the sunnah:
اللهُمَّ اغْفِرْ لِي ذَنْبِي كُلَّهُ دِقَّهُ وَجِلَّهُ وَأَوَّلَهُ وَآخِرَهُ وَعَلَانِيَتَهُ وَسِرَّهُ
O Allah, forgive me for all my sins; minor and major sins, and the first and last sin (i.e. from the first till the last sin), and the sins which I committed openly and secretly.
عن أبي هريرة أن رسول الله صلى الله عليه وسلم كان يقول في سجوده اللهم اغفر لي ذنبي كله دقه، وجله، وأوله وآخره وعلانيته وسره (صحيح مسلم، الرقم: 892)
Hazrat Abu Hurairah (radhiyallahu anhu) reports that Rasulullah (sallallahu alaihi wasallam) would recite the following dua in his sajdah:
اللهُمَّ اغْفِرْ لِي ذَنْبِي كُلَّهُ دِقَّهُ وَجِلَّهُ وَأَوَّلَهُ وَآخِرَهُ وَعَلَانِيَتَهُ وَسِرَّهُ