Musnad Ahmad 538:12

Zayd ibn Yahya al-Dimashqi narrated to us, he said: Abdullah ibn al-Ala narrated to us, he said: I heard Muslim ibn Mishkam, he said: I heard al-Khushani saying: I said: "O Messenger of Allah, inform me of what is lawful for me and what is forbidden for me." He said: The Prophet, peace and blessings of Allah be upon him, looked up and down at me, then said: "Righteousness is that which the soul finds tranquility in and the heart finds peace with, and sin is that which the soul does not find tranquility in and the heart does not find peace with, even if the people give you a legal opinion." And he said: "Do not approach the meat of the domestic donkey, nor any beast of prey that has fangs."