Musnad Ahmad 26:28

Sufyan narrated to us, from ‘Asim, from Abu Wa’il, from ‘Abdullah, who said: We used to greet the Prophet, peace and blessings of Allah be upon him, when we were in Makkah before we came to the land of Abyssinia. When we arrived from the land of Abyssinia, we came to him and greeted him, but he did not return the greeting. So, what was near and what was far [of anxiety] took hold of me, until they finished the prayer. I asked him, and he said: "Indeed, Allah, the Mighty and Majestic, brings about in His command whatever He wills, and He has brought about in His command that we should not speak during the prayer."