Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates: We used to speak during the prayer; one of us would greet another with salam, and one of us would indicate his need to his companion by gesture. One day I came while the Messenger of Allah (peace and blessings be upon him) was praying, and I greeted him with salam, but he did not return my greeting. All my past and recent concerns began to come to my mind. When the Messenger of Allah (peace and blessings be upon him) had finished, he said: "Allah brings about whatever new ruling He wills from His command, and He has brought about the new ruling that you should not speak during the prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10087