Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrated that (previously) when the Prophet Muhammad (peace and blessings be upon him) was performing prayer and we greeted him with salam, he would respond to it. When we returned from Najashi's place, we greeted (as before, during the prayer), but this time he did not respond. Rather, after completing the prayer, he said, "Where is there any opportunity for a person during prayer?"