Riyad us-Salihin Hadith 1029

The Messenger of Allah (ﷺ) went to the (Baqi') cemetery and said, "May you be secured from punishment, O dwellers of abode of the believers! We, if Allah wills, will follow you. I wish we see my brothers." The Companions said, "O Messenger of Allah! Are not we your brothers?" He (ﷺ) said, "You are my Companions, but my brothers are those who have not come into the world yet." They said; "O Messenger of Allah! How will you recognize those of your Ummah who are not born yet?" He (ﷺ) said, "Say, if a man has white-footed horses with white foreheads among horses which are pure black, will he not recognize his own horses?" They said; "Certainly, O Messenger of Allah!" He (ﷺ) said, "They (my followers) will come with bright faces and white limbs because of Wudu'; and I will arrive at the Haud (Al-Kauthar) ahead of them." [Muslim].