Musnad Abu Ya'la al-Mawsili 155:46

And from Abu Hurairah, that a man from Banu Fazarah came to the Prophet, may Allah bless him and grant him peace. He said, "My wife has given birth to a black boy." He said, "Do you have any camels?" He said, "Yes." He said, "What are their colors?" He said, "Red." He said, "Are there any among them that are ashen-colored (dark gray)?" He said, "Yes, there are indeed ashen ones among them." He said, "From where did it get that?" He said, "Perhaps it was pulled by a vein (heredity)." He said, "And this, perhaps it has been pulled by a vein."