Answer: A Muslim should say: "As-salāmu ‘alaykum warahmatullāhi wa barakātuh" (May the peace, mercy, and blessings of Allah be upon you).

His brother should return the greeting, saying: "Wa ‘alaykum as-salām warahmatullāhi wa barakātuh" (And may the peace, mercy, and blessings of Allah be upon you too). [Narrated by At-Tirmidhi, Abu Dāwūd, and others]