Musnad Ahmed: Hadith 3862

I used to greet the Messenger of Allah (ﷺ) when he was praying and he would return my greeting. One day I greeted him (when he was praying) but he did not return the greeting and I felt sad. When he had finished praying I said:O Messenger of Allah, I used to greet you when you were praying and you would return the greeting. He said: "Allah introduces whatever He wants into His matter (i.e., religion)."