Musannaf Ibn Abi Shayba 33:1131

Ali ibn Mushir narrated to us, from Layth, from Abu Ubaydah, who said: He says—meaning Allah, Blessed and Exalted is He—: "What is the matter with a people who seek understanding of religion without worshipping Me? They wear the skins of sheep, while their hearts are more bitter than patience (al-sabir). Are they deluded by Me, or are they trying to deceive Me? By Myself I have sworn: I shall surely settle upon them a trial in the world that will leave the forbearing among them bewildered."