Answer
(Fatwa: 239/192/L=1431)
In a congregational Salah, if a muqtadi commits a mistake requiring sajdah sahv (remedial prostrattion), it does not make sajdah sahv wajib on him. So, in the question mentioned above, if you forgetfully start reciting durood after tashahhud then sajdah sahv is not wajib on you. Yes, if any munfarid (a person performing salah individually) or imam forgetfully starts reciting darood Ibrahimi after at-tahiya?at in first sitting and goes up to (اللهم صل على محمد) then sajdah sahv becomes wajib on him.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband