Musnad Ahmed: Hadith 3552

We used to greet the Prophet (ﷺ) (when he was praying) when we were in Makkah before we went to Abyssinia. When we came from Abyssinia, we came to him and greeted him (when he was praying) but he did not return the greeting and I got very distressed and started trying to find out why, until they finished praying. I asked him and he said: "Allah introduces whatever He wants into His matter (i.e., religion) and He has decreed that we should not speak whilst praying.”