Imam al-Shafi'i (may Allah have mercy on him) said: "'And if you disagree over anything' [Surah al-Nisa: 59] means, and Allah knows best: they and their rulers whom they have been commanded to obey. 'Refer it to Allah and the Messenger' [Surah al-Nisa: 59] means, and Allah knows best: to what Allah and the Messenger have said."
And Allah Almighty said: "Does man think that he will be left neglected (sudan)?" [Surah al-Qiyamah: 36]
Imam al-Shafi'i (may Allah have mercy on him) said: "To my knowledge, the scholars of the Qur'an do not differ that 'sudan' is the one who is neither commanded nor forbidden anything. And whoever gives a fatwa or a judgement on something he was not commanded to, has permitted himself to fall within the meaning of 'sudan' (one left neglected)."
The Shaykh (Imam al-Bayhaqi) (may Allah have mercy on him) said: "And it has been narrated to us from Imam Mujahid (may Allah have mercy on him), in the interpretation of these two verses, something similar to what Imam al-Shafi'i (may Allah have mercy on him) said."
Yazid bin Hayyan said: I heard Sayyiduna Zayd bin Arqam (may Allah be pleased with him) say: One day the Prophet (peace and blessings be upon him) stood among us delivering a sermon. After praising and extolling Allah, he (peace and blessings be upon him) said: "O people, I am only a human being. Soon the messenger of my Lord will come and I will respond to him (meaning death). I am leaving among you two things: the first is the Book of Allah, in which there is guidance and light. So hold fast to the Book of Allah." Then he (peace and blessings be upon him) urged (adherence to) the Book of Allah and encouraged it. Then he said: "The second is my household; I exhort you regarding my household," three times.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20335