Musnad Abu Ya'la al-Mawsili 152:121

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