Silsilat al-Ahadith al-Sahihah

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."