Musnad Ahmad 26:335

Muhammad ibn Fudayl narrated to us, Mutarrif narrated to us, from Abu al-Jahm, from Abu al-Ridrad, from Abdullah ibn Mas’ud, who said: I used to greet the Messenger of Allah (peace and blessings of Allah be upon him) while he was in prayer, and he would return the greeting to me. Then, one day, I greeted him, and he did not return the greeting to me, so I felt distressed within myself. When he finished, I said: "O Messenger of Allah, when I used to greet you in prayer, you would return it to me?" He said: "Indeed, Allah, the Almighty and Majestic, brings about in His command whatever He wills."