It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him): We used to give salam and speak about our needs during the prayer. I came (on my return from Abyssinia) to the Messenger of Allah (peace and blessings be upon him) while he (peace and blessings be upon him) was praying. I greeted him with salam, but he (peace and blessings be upon him) did not return my greeting. I was overcome with worry about things old and new. When the Messenger of Allah (peace and blessings be upon him) had completed the prayer, he said: "Allah Almighty sends down a new command whenever He wills, and now the command of Allah Almighty is that you must not talk during the prayer." Then he (peace and blessings be upon him) returned my greeting.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3410