The Messenger of Allah (may Allah's peace and blessings be upon him) went out to Qubā' Mosque to pray in it

‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) went out to Qubā' Mosque to pray in it. He added: "Some of the Ansār came to him and greeted him while he was praying." He then added: "I asked Bilāl: "How did you see the Messenger of Allah (may Allah's peace and blessings be upon him) return their greeting when they greeted him while he was praying?" Bilāl said: "He made this gesture,"

and he stretched out his hand with the palm facing down and the back facing up.

This noble Hadīth clarifies the permissibility of returning the greeting with a hand gesture during prayer based on the action of the Prophet (may Allah's peace and blessings be upon him) with the Ansār; when they greeted him while he was praying in Qubā' Mosque, he simply extended his hand.