The Messenger of Allah (may Allah's peace and blessings be upon him) went out to Qubā' Mosque to pray in it
Authentic hadithعن عبد الله بن عمر رضي الله عنهما قال: «خرج رسول الله صلى الله عليه وسلم إلى قُبَاءَ يُصَلِّي فيه»، قال: «فَجَاءَتْه الأنصار، فَسَلَّمُوا عليه وهو يُصلِّي»، قال: " فقلت لبِلاَل: كيف رأَيْت رسول الله صلى الله عليه وسلم يَرُدُّ عليهم حِين كانوا يُسَلِّمُونَ عليه وهو يُصلِّي؟ "، قال: يقول هَكَذا، وبَسَطَ كفَّه، وجعل بطنه أسفل، وجعل ظَهْرَه إلى فوْق.
‘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.
Explanation
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.
Source · Narrated by At-Termedhy - Narrated by Abu Daoud - Narrated by Ahmad
From the Hadith Encyclopedia (hadeethenc.com).