Abu Hurairah (May Allah be pleased with him) reported:
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].

وعنه أن رسول الله ﷺ أتى المقبرة فقال: "السلام عليكم دار قوم مؤمنين وإنا إن شاء الله بكم لاحقون، وددت أنا قد رأينا إخواننا" قالوا: أولسنا إخوانك يا رسول الله؟ قال: "أنتم أصحابي، وإخواننا الذين لم يأتوا بعد" قالوا: كيف تعرف من لم يأتِ بعد من أمتك يا رسول الله؟ فقال: "أرأيت لو أن رجلا له خيل غر محجلة بين ظهري خيل دهم بهم، ألا يعرف خيله ؟" قالوا بلى يا رسول الله، قال: فإنهم يأتون غرًا محجلين من الوضوء وأنا فرطهم على الحوض ((رواه مسلم)).