Yahya related to me from Malik from Nafi that Abdullah ibn Umarpassed by a man who was praying and said, "Peace be upon you," and theman replied to him. Abdullah ibn Umar returned to him and said, "Whensomeone says, 'Peace be upon you' to you while you are praying do notreply, but give a signal with your hand."
USC-MSA web (English) reference: Book 9, Hadith 79
Arabic reference: Book 9, Hadith 410