Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) came to (the mosque of) Quba. He (peace and blessings be upon him) was offering prayer when some Ansari people came and greeted him with salam. I asked Sayyiduna Bilal (may Allah be pleased with him): When they greeted him with salam while he (peace and blessings be upon him) was in prayer, how did he (peace and blessings be upon him) reply to their salam? He said: He (peace and blessings be upon him) would do like this. Then he spread his palm. (While describing this manner) Ja'far bin 'Awn spread his palm and kept its inner side downward and the outer side upward.