Musannaf Ibn Abi Shayba 33:69

Abu Usama narrated to us, from Abu al-Ashhab, who said: Khalid ibn Thabit al-Rab'i narrated to me—and Ja'far, who used to read the books—that Luqman was an Ethiopian slave who was a carpenter. His master said to him, "Slaughter a sheep for me." He slaughtered a sheep for him. He said, "Bring me its two best morsels." So he brought him the tongue and the heart. He said, "Was there nothing in it better than these two?" He said, "No." Then he remained silent for as long as he remained silent. Then he said, "Slaughter a sheep for me." He slaughtered a sheep for him. He said, "Discard its two worst morsels." So he discarded the tongue and the heart. He said to him, "I told you to bring me its two best morsels, and you brought me the tongue and the heart. Then I told you to discard its two worst morsels, and you discarded the tongue and the heart." He said, "There is nothing better than these two if they are good, and nothing worse than these two if they are corrupt."