Abu Dawud Book 2, Hadith Number 927.
Chapter : Not known.
Narated By Abdullah ibn Umar : The Apostle of Allah (PBUH) went to Quba to offer prayer. Then the Ansar (the Helpers) came to him and gave him a salutation while he was engaged in prayer.
I asked Bilal: How did you find the Apostle of Allah (PBUH) responding to them when they gave him a salutation while he was engaged in prayer. He replied: In this way, and Ja’far ibn Awn demonstrated by spreading his palm, and keeping its inner side below and its back side above.