Answered by Shaykh Faraz Rabbani
If one who is praying alone Fajr, Maghrib, or Isha, and is reciting silently… and someone comes up and taps him on the shoulder and joins him in prayer, when should the imam raise his voice? Should he raise it immediately even if in the middle of an ayah? or should he remain reading silently, and wait until the next rakaat to read aloud (if it’s the second rakaat)?
Answer:
Walaikum assalam,
One should start reciting out loud immediately, as the rulings of being an imam now apply to one.
Walaikum assalam,
Faraz Rabbani.