It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever tells a lie about me, let him take his seat in Hell. Whoever's brother seeks his advice and he does not give him sound advice has betrayed him. And whoever gives a fatwa without proof, the sin is upon the one who gave the fatwa."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20324