Musnad Abu Ya'la al-Mawsili 154:13

Abu Khaythama narrated to us, Ibn 'Uyaynah narrated to us, Zayd ibn Aslam narrated to us, from 'Abdullah ibn 'Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) entered the Mosque of Quba—which is the mosque of the Banu 'Amr ibn 'Awf—to pray in it. Men from the Ansar entered upon him and greeted him while he was in the prayer. I asked Suhayb, who had entered with him, "How did the Prophet (may Allah bless him and grant him peace) do when he was greeted while in prayer?" He said, "He would gesture with his hand."