It was narrated that Ibn Mas'ood said:

I used to greet the Prophet (ﷺ) with salam when he was praying and he would return the greeting. Then I greeted him with salam one day and he did not say anything to me in response, and I felt upset. I said: O Messenger of Allah, I used to greet you with salam when you were praying and you would return my greeting. Then I greeted you with salam and you did not say anything to me in response. The Messenger of Allah (ﷺ) said: “Verily Allah introduces into His matter (i.e. religion) whatever He wants."