Qays, the freed slave of Sayyiduna Khabbab (may Allah be pleased with him), said: "I saw Sayyiduna al-Hasan and Sayyiduna al-Husayn (may Allah be pleased with them both) dyeing their hair black."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2769