Musnad Abu Ya'la al-Mawsili 160:107

And with his chain of narration, that the Messenger of Allah (ﷺ) came to the graveyard and 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 our brothers are those who have not yet come." They said: "How will you recognize those who have not yet come from your nation, 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, and I am their forerunner to the Basin (Hawd). Beware, some people 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 [the religion] after you." Then I will say: "Away, away!"