Q: Must one make sujood sahu even when you ommitted a sunnah of salaah, e.g. a reciting a surah after surah Fatiha? What about ommitting the a wajib such as the qunoot in Witr?
A: Sajda sahu is made when one leaves out a waajib. Note that to read a Surah after Surah Faatiha in all rakaats except the third and fourth rakaat of faradh salaah, is also waajib. so if one left out a Surah after Surah Faatiha then sajda sahu must be made.
Omitting Qunoot of witr also necessitates sajda sahu because the dua of Qunoot is waajib.
Mufti Siraj Desai