Sayyiduna Ibn Mas'ud (may Allah be pleased with him) narrates that in the beginning, we used to greet the Prophet (peace and blessings be upon him) with salam during prayer, and he would respond to us. But when we returned from Najashi and greeted him with salam, he did not respond. We said: O Messenger of Allah (peace and blessings be upon him)! Previously, we used to greet you with salam during prayer and you would respond to us? The Prophet (peace and blessings be upon him) said: "Indeed, there is engagement in prayer."