Musnad Ahmad 30:875

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, he said: I heard al-‘Ala’ ibn ‘Abd al-Rahman narrating from his father, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him): That he came to the graveyard and greeted the people of the graveyard, saying: "Peace be upon you, O abode of a believing people, and we, if Allah wills, will be joining you." Then he said: "I wish that we had seen our brothers." They said: "O Messenger of Allah, are we not your brothers?" 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: "O Messenger of Allah, how will you recognize those of your nation who have not yet come?" He said: "Do you see, if a man had horses with white foreheads and white feet among horses that were entirely black, would he not recognize them?" They said: "Yes." He said: "For they will come on the Day of Resurrection with white foreheads and white feet from the traces of ablution, and I am their forerunner to the Basin." Then he said: "Verily, men from among you 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.' So I will say: 'Away, away!'"