Maliks Muwatta Book 9, Hadith Number 79.

Section : How to Do the Prayer in General.

Yahya related to me from Malik from Nafi that Abdullah ibn Umar passed by a man who was praying and said, “Peace be upon you,” and the man replied to him. Abdullah ibn Umar returned to him and said, “When someone says, ‘Peace be upon you’ to you while you are praying do not reply, but give a signal with your hand.”