Praise be to Allah.
What is to be said in Sujud As-Sahw is the same as what is to be said in the sujud of prayer, which is to say Subhana Rabbi al-A’la (Glory be to my Lord Most High) or other tasbihat which are reported as being sunnah to say in sujud. Other du`as which are sunnah to say in sujud may also be recited.
There is no specific mention of how the tasbih should be said in Sujud As-Sahw . The author of Al-Sunan wal-Mubtada’at said:
“There is no report that the Prophet (peace and blessings of Allah be upon him) said any specific words in Sujud As-Sahw ; the adkhar to mentioned in Sujud As-Sahw are the same as those to be recited in the sujud of prayer.
As regards the belief that one should say Subhan man la yas-hu wa la yanam (Glory be to the One Who does not forget or sleep), neither the Prophet (peace and blessings of Allah be upon him) nor the Sahabah did this, and there is no evidence for it in the Sunnah. (Pp. 74-75)
And Allah knows best.