asalam o alikum I know how to do sajda sahu within the namaz.But sometimes when i am about to finish namaz ,in the last rakats like the last rakat of zuhar,asar or maghrib etc.i forgot a surah or something and i did sajda sahu after salam like I said my salams on both shoulders,then i sat and said tashhood and then salam on right shoulder and then alahoakbar and then i did sadjas and then sat and said tashood and darood and duas and then salam on both shoulders and finish namaz.Was this correct. coz i didnot know that in last rakat when you finish namaz and forget something and also forget the sajda sahu then i think i can do like this.I also didnot ask anyone,coz i thought i was right were my prayers valid,please tell me or i have to say them again. jazakallah

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It appears from the question that the questioner knows how to prostrate for forgetfulness before concluding the prayer (before saying Salaam), and that she is asking how to prostrate for forgetfulness after concluding the prayer. We have already issued Fatwa 89411 in this regard, so please refer to it.

As regards one who forgets to prostrate for forgetfulness before saying Salaam (concluding the prayer) and concluded the prayer, and only remembered after saying Salaam, then he should prostrate for forgetfulness and there is no harm on him.

Al-Buhooti from the Hanbali school said: 'If one forgets to prostrate for forgetfulness which he should have done before concluding the prayer, and only remembered after concluding the prayer, then it is an obligation on him to prostrate for forgetfulness if the time between him remembering it and concluding the prayer is not too long.' For more benefit on whoever forgets to perform the prostration for forgetfulness, please refer to Fatwa 90941, and refer to Fatwa 81875 about the obligations which require prostrating for forgetfulness. 

Allaah Knows best.