Musnad Ahmad 26:394

Asbat and Ibn Fudayl narrated to us—the meaning being the same—they said: Mutarrif narrated to us, from Abu al-Jahm, from Abu al-Ridrad, from Ibn Mas’ud, who said: I used to greet the Prophet, peace and blessings of Allah be upon him, while he was in prayer, and he would return the greeting to me. Then I greeted him one day, and he did not return anything to me, so I felt something in myself. I said: "O Messenger of Allah, I used to greet you while you were in prayer, and you would return the greeting to me, but I greeted you and you did not return anything to me?" The Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, Allah brings about in His command whatever He wills."