When performing the prostrations of forgetfulness and of recitation, you should say the same as you say when you prostrate in prayer: “Subhaana Rabbiy al-A’la (Glory be to my Lord Most High).” What is obligatory is to say that once, and the least required in order to do it perfectly is to say it three times. It is mustahabb to make du’aa’ when prostrating, reciting whatever you can of the du’aa’s prescribed in sharee’ah, because the Prophet (peace and blessings of Allaah be upon him) said: “When bowing glorify your Lord, and when prostrating strive in making du’aa’, for then you are more likely to receive an answer.”
And the Prophet (peace and blessings of Allaah be upon him) said: “The closest that a person can be to his Lord is when he is prostrating, so say a lot of du’aa’.”
These two hadeeth were both narrated by Muslim in his Saheeh. The Prophet (peace and blessings of Allaah be upon him) often used to say when bowing and prostrating: “Subhaanaka Allaahumma wa bi hamdika, Allaahumma ighfir li (Glory be to You, O Allaah, and praise. O Allaah, forgive me).” Agreed upon – from the hadeeth of ‘Aa’ishah (may Allaah be pleased with her). And he (peace and blessings of Allaah be upon him) also used to say when bowing and prostrating: “Subbooh quddoos Rabb ul-malaa’ikah wa’l-rooh (Glorified, Holy, Lord of the angels and the Spirit [i.e., Jibreel].” Narrated by Muslim in his Saheeh.
And Allaah is the Source of strength.