Sahih Bukhari: Hadith 6169

The Jews used to greet the Prophet (ﷺ) by saying, "As-Samu 'Alaika (i.e., death be upon you), so I understood what they said, and I said to them, "As-Samu 'alaikum wal-la'na (i.e. Death and Allah's Curse be upon you)." The Prophet (ﷺ) said, "Be gentle and calm, O `Aisha, as Allah likes gentleness in all affairs." I said, "O Allah's Prophet! Didn't you hear what they said?" He said, "Didn't you hear me answering them back by saying, 'Alaikum (i.e., the same be upon you)?"