Musnad Ahmad 30:2281

Qutaybah ibn Sa'id narrated to us, 'Abd al-'Aziz ibn Muhammad narrated to us, from Abu Thifal al-Murri, from Rabah ibn 'Abd al-Rahman, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The blood of an 'Afra' (a white-colored animal with a reddish tint) is more beloved to me than the blood of two black ones."