Sahih Muslim Book 4, Hadith Number 1098.

Chapter : Sitting on the buttocks.

Zaid b. Arqam reported: We used to talk while engaged in prayer and a person talked with a companion on his side in prayer till (this verse) was revealed: “And stand before Allah in devout obedience” (ii, 238) and we were commanded to observe silence (in prayer) and were forbidden to speak.