Musnad Ahmad 32:1403

Ali ibn Ishaq narrated to us, Abdullah informed us, he said: Yunus informed us, from al-Zuhri, he said: Anas ibn Malik informed me, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said to the Ansar: "You will surely encounter intense selfishness [after me], so be patient until you meet Allah and His Messenger, for I will be at the Hawd (the Cistern)." They said: "We will be patient." Abdullah said: "If Allah wills," and he lowered his voice, so I thought it was not part of the hadith.