بسم الله الرحمن الرحيم
Q: Is the tasleem said simultaneously with the imam or after him? Also is the second tasleem voluntary or obligatory?
A: The tasleem is after the imam, however some hold the opinion that the imam concludes the prayer with the first tasleem. Ending the prayer is by way of the first tasleem and the second is voluntary, as some narrations only mention one tasleem. So if a person says the tasleem after the (imam’s) first tasleem, for instance the imam says “as salaamu alaykum wa rahmatullah” and thereby completes the first tasleem, then some followers say “as salaamu alaykum wa rahmatullah” similar to him, they have not opposed the Sunnah, if Allah wills.
Translated by Abu Abdil Waahid Najeeb Al Anjelesi
Source: فتاوى فضيلة الشيخ العلامة ربيع بن هادي عمير المدخلي