Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) that a man greeted the Prophet Muhammad (peace and blessings be upon him) with salam while he was in prayer. The Prophet (peace and blessings be upon him) replied to his salam by gesture. When the Prophet (peace and blessings be upon him) finished the prayer, he said: "We used to reply to salam during prayer, but now we have been forbidden from doing so."

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he used to greet the Messenger of Allah (peace and blessings of Allah be upon him) with salam while he was praying, and the Prophet (peace and blessings of Allah be upon him) would respond to him. (One day) he greeted with salam, but the Prophet (peace and blessings of Allah be upon him) did not respond. Sayyiduna Abdullah (may Allah be pleased with him) thought that the Messenger of Allah (peace and blessings of Allah be upon him) was upset with him. When the Prophet (peace and blessings of Allah be upon him) finished the prayer, he asked: O Messenger of Allah! I used to greet you with salam during prayer and you would respond to me, but today I greeted you and you did not respond. I thought you were upset with me. The Prophet (peace and blessings of Allah be upon him) said: "It is not so. In fact, we have been forbidden from engaging in any action during prayer except for the recitation of the Qur'an and the remembrance (dhikr) of Allah."

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.