If the one who is praying behind an imam says the tasleem before the imam
Question

If one of those who are praying behind an imam says one tasleem to the right before the imam, by mistake, does that render his prayer invalid?
Praise be to Allah.His prayer is not rendered invalid, but he should say the tasleem again after the imam says it. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)
Fataawa Noor ‘ala ad-Darb (2/936).