Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: We used to greet the Messenger of Allah (peace and blessings be upon him) with salam while he was in prayer, and he would respond to us. But when we returned from Najashi and greeted the Messenger of Allah (peace and blessings be upon him) with salam, he did not respond to us. We said: O Messenger of Allah! We used to greet you with salam during prayer and you would respond to us (but today?). The Messenger of Allah (peace and blessings be upon him) said: Indeed, there is engagement in prayer.