Musnad Ahmad 26:595

'Abd al-Rahman narrated to us, Za'idah narrated to us, from 'Asim, from Shaqiq, from 'Abdullah, who said: We used to speak during the prayer, and we would greet one another, and one of us would give instructions regarding a need. So I came to the Prophet, peace and blessings of Allah be upon him, and I greeted him while he was praying, but he did not return the greeting to me. Then, what had passed and what had occurred [of anxiety] took hold of me. When he finished praying, he said: "Indeed, Allah, the Almighty and Majestic, brings about whatever He wills of His command, and He has brought about that you should not speak during the prayer."