Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates: I came (from Abyssinia) into the presence of the Messenger of Allah (peace and blessings be upon him), and 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,) I said: O Messenger of Allah, has some new command come? The Messenger of Allah (peace and blessings be upon him) said: "Allah Almighty sends down for His Prophet whatever new command He wills. Now the new command of Allah is that you should not talk in prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3345