Musnad Ahmad 30:378

Yazid narrated to us, Muhammad informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "People are like mines; you will find the best of them in the Pre-Islamic period (Jahiliyyah) to be the best of them in Islam, if they attain understanding (fiqh)."