Musnad Ahmad 2:185

'Affan narrated to us, Hammad ibn Salamah narrated to us, from Sa'id al-Jurayri, from Abu Nadrah, from Usayr ibn Jabir, who said: When the people of Yemen arrived, 'Umar began to inquire about the groups, saying: "Is there anyone among you from Qarn?" Until he came to Qarn and said: "Who are you?" They said: "Qarn." Then the rein of 'Umar—or the rein of Uways—fell, and one of them handed it to the other, and he recognized him. 'Umar said: "What is your name?" He said: "I am Uways." He said: "Do you have a mother?" He said: "Yes." He said: "Did you have any whiteness (leprosy/vitiligo)?" He said: "Yes, so I supplicated to Allah, the Mighty and Majestic, and He removed it from me, except for the place of a dirham on my navel, so that I may remember my Lord by it." 'Umar said to him: "Ask forgiveness for me." He said: "You are more entitled to ask forgiveness for me; you are a companion of the Messenger of Allah, peace and blessings of Allah be upon him." 'Umar said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Indeed, the best of the Tabi'in is a man who is called Uways, and he has a mother, and he had whiteness, so he supplicated to Allah, the Mighty and Majestic, and He removed it from him, except for the place of a dirham on his navel.'" So he asked forgiveness for him, then he entered into the crowd of people, and it was not known where he went. He said: So he arrived in Kufa. He said: We used to gather in a circle and remember Allah, and he would sit with us. When he spoke, his speech would settle in our hearts in a way that the speech of no one else would... then he mentioned the rest of the hadith.