Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that I used to greet the Prophet (peace be upon him) with salam during prayer and he would respond to me. But one day I greeted him with salam and he did not respond. This saddened me greatly. When the Prophet (peace be upon him) finished the prayer, I said: O Messenger of Allah (peace be upon him)! Previously, when I greeted you during prayer, you would respond to my salam? He replied: "Allah the Exalted sends new commands in His matters as He wills."