بسم الله الرحمن الرحيم
(Fatwa: 531/415/B=1431)
Salah completes on the mim of assalamu. If you joined Imam by saying takbir tahrima before he pronounced mim you will be counted as participant of Jamaat, but not if you joined after the pronunciation of the letter mim.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband
