The Prophet (peace and blessings be upon him) said: "I have not left anything that Allah commanded you except that I have commanded you with it, and I have not left anything that Allah Almighty forbade you except that I have forbidden it to you."
Imam al-Shafi'i (may Allah have mercy on him) said: So in whatever had no revelation, Allah has made it obligatory in the revelation to follow his Sunnah; so whoever accepts from him accepts only by the obligation laid down by Allah, the Mighty and Majestic. And Allah, Glorified and Exalted, knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13443