Musnad Ahmad 31:262

Muhammad ibn Abdullah ibn al-Zubayr narrated to us, Kathir ibn Zayd narrated to us, from Rabih ibn Abd al-Rahman ibn Abi Sa’id al-Khudri, from his father, from his grandfather, who said: We used to take turns [serving] the Messenger of Allah, peace and blessings of Allah be upon him, and we would spend the night with him in case he had a need or a matter arose for him during the night, so he would send us. The number of those seeking reward and those on duty would increase, and we would talk. The Messenger of Allah, peace and blessings of Allah be upon him, came out to us during the night and said: "What is this whispering? Did I not forbid you from whispering?" We said: "We repent to Allah, O Prophet of Allah. We were only speaking about the Messiah (the Antichrist), fearing him." He said: "Shall I not inform you of what is more fearful to me for you than the Messiah?" We said: "Yes." He said: "The hidden shirk: that a man stands and performs deeds for the sake of a man."