Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: We used to greet the Prophet (peace and blessings be upon him) with salam while he was in prayer, and he would return our greeting. Then, when I came from the land of Abyssinia, I greeted him with salam, but he did not return my greeting. All my past and recent concerns began to come to my mind. Then I waited for him, and when he had finished the prayer, I mentioned it to him, and he said: "Allah brings about whatever new ruling He wills from His command, and He has decreed" - or he said: "He has brought about a new ruling" - "that you should not speak during the prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10086