From Sahih Bukhari (Hadith 3544):
Narrated Isma`il bin Abi Khalid:I heard Abu Juhaifa saying, "I saw the Prophet, and Al-Hasan bin `Ali resembled him." I said to Abu- Juhaifa, "Describe him for me." He said, "He was white and his beard was black with some white hair. He promised to give us 13 young she-camels, but he expired before we could get them
Reference: Bukhari 3544