The Prophet (peace and blessings be upon him) said: "Marriage is not lawful without the permission of a guardian, nor without a dower and two truthful and upright witnesses."
Al-Shafi'i (may Allah have mercy on him) said: Even though this is munqati' (broken) short of the Prophet (peace and blessings be upon him), most of the people of knowledge hold this view, and they say: The difference between marriage and fornication is the witnesses.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13720