Musannaf Ibn Abi Shayba 3:2426

Sufyan ibn 'Uyaynah narrated to us, from 'Asim, from Abu Wa'il, from 'Abdullah, who said: We used to greet the Prophet, may Allah bless him and grant him peace, while he was praying, and he would return the greeting to us before we went to the land of Abyssinia. When we returned from the land of Abyssinia, I greeted him, but he did not return the greeting to me, so I was overcome by what was near and what was far (i.e., I was deeply distressed and anxious). When he finished his prayer, he said: "Indeed, Allah brings about whatever He wills of His command, and He has brought about that you should not speak during the prayer."