Musnad Ahmad 30:1147

Abdullah ibn Yazid narrated to us, from his book, he said: Sa’id—meaning Ibn Abi Ayyub—narrated to me, Bakr ibn ‘Amr al-Mu’afiri narrated to me, from ‘Amr ibn Abi Nu’aym, from Abu ‘Uthman Muslim ibn Yasar, from Abu Hurayrah, who said: The Messenger of Allah (ﷺ) said: "Whoever attributes to me what I have not said, let him take his seat in the Fire. And whoever is consulted by his Muslim brother, and he advises him with something other than what is correct, he has betrayed him. And whoever gives a legal opinion (fatwa) without a firm basis, his sin is only upon the one who gave him the opinion."