Question Summary:
what if I recited by mistake a part or one ayah after surah fatiha in third or fourth rakah in farad prayer. I have to complete the surah after fatiha or you just go to ruku and then make sajda sawhu. please kindly explain details about sajda sawhu. and in what condition you have to start over.

Question Detail:

how to do sajda sawhu?

what if I recited by mistake a part or one ayah after surah fatiha in third or fourth rakah in farad prayer. I have to complete the surah after fatiha or you just go to ruku and then make sajda sawhu. please kindly explain details about sajda sawhu. and in what condition you have to start over.  
if I forget dua-e-qanoot in my witar and went to ruku do I have to make witar prayer again? or just do the sajda sawho.

Answer :

In the name of Allah, Most Gracious, Most Merciful
Assalaamu `alaykum waRahmatullahi Wabarakatuh
In the third and fourth rakats of farz salah, it is Afzal (more virtuous) to recite sura Fatiha. One should not recite a sura after sura Fatiha. However if one recited an ayah or more after sura Fatiha, that will not necessitate sajda sahw.
Sajda sahw is performed to compensate an unintentional omission of a wajib act of salah, for example, one did not recite sura Fatiha, or did not recite an additional part of the Quran after sura Fatiha or one did not sit in tashahhud after two rakats in a four rakats salah etc.
Dua-e-Qunoot in witr is wajib. If one went into ruku without reading Due-e-Qunoot, then one should continue with the witr and make sajda sahw at the end.
And Allah knows best
Wassalam
Mufti Ebrahim Desai
Darul Iftaa, Madrassah In'aamiyyah