Answer:
(Fatwa: 128/128/M=1432)
(1) At this time instead of reading Sallallahu Alaihe Wasallam, the same kalimah shahadah should be repeated.
(2) You can read in your heart.
(3) If one recites durood in the last rakah of salah by mistake it does not make sajdah sahv wajib. Yes, if after Imam has turned salam, and one stood to complete one’s missed salah and he commits any act that makes sajdah sahv wajib, then sajdah sahv shall be necessary.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband
