Musnad Ahmad 30:1655

Yahya ibn Ghaylan narrated to us, he said: Rishdin narrated to us, Bakr ibn Amr narrated to me, from Amr ibn Abi Nu'aym, from Abu Uthman, the companion of Abu Hurayrah, from the Messenger of Allah, peace and blessings of Allah be upon him, that he said: "Whoever says about me what I have not said, let him take his seat in the Fire. And whoever gives a legal opinion (fatwa) without knowledge, the sin of that is upon the one who gave him the opinion. And whoever consults his brother, and he advises him with a matter, while he sees that the right course is other than that, then he has betrayed him."