Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates: We used to give greetings, speak and talk to one another about our needs in prayer. But when (on returning from Abyssinia) I came one day into the presence of the Prophet (peace and blessings be upon him), he (peace and blessings be upon him) was praying at that time. I greeted him (peace and blessings be upon him), but he (peace and blessings be upon him) did not return the greeting. I was seized with worry over old and new matters. When the Messenger of Allah (peace and blessings be upon him) had completed the prayer, he said: "Allah Almighty sends down whatever new command He wills. Now the new command of Allah Almighty is that you should not talk in prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3346