Musnad Ahmad 30:2169

'Affan narrated to us, he said: 'Abd al-Rahman ibn Ibrahim narrated to us, he said: al-'Ala' ibn 'Abd al-Rahman narrated to us, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) went out to the graveyard and greeted its inhabitants. He said: "Peace be upon you, O abode of a believing people, and we, if Allah wills, will join you. I wish that we had seen our brothers." They said: "Are we not your brothers, O Messenger of Allah?" He said: "Rather, you are my companions, and my brothers are those who have not yet come, and I am their forerunner to the Basin (al-Hawd)." They said: "How will you recognize those of your nation who have not yet come, O Messenger of Allah?" He said: "Do you see, if a man had horses with white foreheads and white feet among horses that are solid black, would he not recognize his horses?" They said: "Yes, O Messenger of Allah." He said: "For they will come with white foreheads and white feet from the ablution (wudu)"—he said it three times—"and I am their forerunner to the Basin. Beware, men will be driven away from my Basin just as a stray camel is driven away. I will call out to them: 'Come here!' It will be said: 'They changed things after you.' So I will say: 'Away, away!'"