Musnad Abu Ya'la al-Mawsili 151:5

Abu Khaythama narrated to us, Sufyan ibn 'Uyayna narrated to us, from 'Asim, from Abu Wa'il, who said: 'Abdullah said: "We used to greet the Prophet, peace and blessings of Allah be upon him, and he would return the greeting to us, meaning during the prayer. Then, when we came from the land of Abyssinia, I greeted him, but he did not return the greeting to me. So, what was far and what was near took hold of me (i.e., I became overwhelmed with anxiety). I sat until, when he finished the prayer, I said to him: 'You used to return the greeting to us.' He said: 'Indeed, Allah brings about what He wills, and He has brought about in His command a decree that you should not speak during the prayer.'"