Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates: I came to the Prophet (peace and blessings be upon him) while he was praying, and I greeted him with salam, but he did not return my greeting. When he had finished his prayer, he said: "Allah, the Mighty and Majestic, brings about for His Prophet whatever new ruling He wills, and among the new rulings He has laid down is that you should not speak during the prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10084