Sunna Notes Volume 3: The Binding Proof of the Sunnah

"The meaning of the expression Binding Proof of the Sunnah is that the Sunna points to the judgment of Allah Most High, either as definitive knowledge ('ilm) or as as probable knowledge (zann), bringing it out and disclosing it for us. We understand the Divine rulings by means of the Sunna and it becomes binding upon us to put its content into practice, So the real meaning of the Sunna as binding proof is exposition and proof-inference necessitating obligatory practice of their results because the latter is the Divine decision."
Dr. 'Abd al-Ghani B, 'Abd al-Khaliq
"The Sunna is the first subsidiary source (masadar far i) after the Quran among the sources of Islamic Law. We describe it as a subsidiary source because it is the Quran that actually indicated that it is a source, and the basis of everything is the Quran. The Quran commanded us to follow the Messenger of Allah: therefore we follow the Messenger of Allah."
Dr. Muhammad Sa'id Ibn Mulla Ramadan Al-Buti