Musnad Ahmad 30:3825

Kathir narrated to us, Ja'far narrated to us, Yazid ibn al-Asamm narrated to us, from Abu Hurairah, from the Messenger of Allah (peace and blessings of Allah be upon him)—and Kathir said once: a hadith he attributed [to the Prophet]—he said: "People are like mines, like the mines of gold and silver. The best of them in the Pre-Islamic Period (Jahiliyyah) are the best of them in Islam, if they attain understanding (fiqh). And souls are conscripted soldiers; those among them that recognize one another will become harmonious, and those among them that do not recognize one another will differ."