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:
أللهُمَّ أَعُوذُ بِرِضَاكَ مِنْ سَخَطِكَ، وَبِمُعَافَاتِكَ مِنْ عُقُوبَتِكَ، وَأَعُوذُ بِكَ مِنْكَ لَا أُحْصِي ثَنَاءً عَلَيْكَ أَنْتَ كَمَا أَثْنَيْتَ عَلَى نَفْسِكَ
O Allah, I seek refuge in Your pleasure from Your displeasure, and in Your forgiveness from Your punishment, and I seek refuge in You (in Your mercy) from You (from Your punishment). I am unable to praise You as You ought to be praised. You are as You have praised Yourself.
عن عائشة، قالت: فقدت رسول الله صلى الله عليه وسلم ليلة من الفراش فالتمسته فوقعت يدي على بطن قدميه وهو في المسجد وهما منصوبتان وهو يقول: اللهم أعوذ برضاك من سخطك، وبمعافاتك من عقوبتك، وأعوذ بك منك لا أحصي ثناء عليك أنت كما أثنيت على نفسك (صحيح مسلم، الرقم: 486)
It is reported that Hazrat Aaishah (radhiyallahu anha) said, “One night (when I awoke), I did not find Rasulullah (sallallahu alaihi wasallam) sleeping on the bed, so I began to search for him. Thereafter, my hand touched the bottom of his mubaarak feet while he was in the position of sajdah and his feet were upright. He was reciting the following dua (while in the position of sajdah):
أللهُمَّ أَعُوذُ بِرِضَاكَ مِنْ سَخَطِكَ، وَبِمُعَافَاتِكَ مِنْ عُقُوبَتِكَ، وَأَعُوذُ بِكَ مِنْكَ لَا أُحْصِي ثَنَاءً عَلَيْكَ أَنْتَ كَمَا أَثْنَيْتَ عَلَى نَفْسِكَ