Ikrimah (may Allah have mercy on him) narrates from Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) delivered a sermon on the occasion of the Farewell Hajj and said: "O people, I am leaving among you that which, if you hold fast to it, you will never go astray: 1. the Book of Allah, 2. the Sunnah of His Prophet (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20336