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."