A group of Jews asked permission to visit the Prophet (and when they were admitted) they said, "As-Samu 'Alaika (Death be upon you)." I said (to them), "But death and the curse of Allah be upon you!" Allah's Messenger (sallallaahu 'alaihi wasallam)said, "O 'Aisha! Allah is kind and lenient and likes that one should be kind and lenient in all matters." I said, "Haven't you heard what they said?" He said, "I said (to them), 'Wa 'Alaikum (and upon you).'

Sahih Al-Bukhari Vol. 9 : No. 61 Narrated by Aisha (Radhiallaho anha)