Mukhtasar Bukhari

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says that we used to greet the Prophet (peace and blessings of Allah be upon him) with salam while he was in prayer, and he (peace be upon him) would also reply to us. Then when we returned from Najashi (the king of Abyssinia), we greeted the Prophet (peace be upon him) with salam during prayer, but he did not reply to us. After completing the prayer, he said: In prayer, one is engaged (with Allah). Therefore, during prayer, one should not be engaged in anything else.