Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Abdur Rahman bin Yazid that one day people came to Hazrat Abdullah, and he said: O people! Indeed, there was a time upon us when neither did we pass judgments nor were we present at the place of judgment. Then Allah Almighty destined the matter of leadership for us, as you see. So after today, whoever among you is presented with the responsibility of judgment, he should judge according to the Qur'an, and if a matter arises which is not in the Qur'an, then he should judge according to the Sunnah of the Noble Prophet (peace and blessings be upon him), and if a matter arises which is neither in the Qur'an nor in the Hadith, then judge according to what the righteous have decided, and if there is a matter which is neither in the Qur'an nor in the Sunnah, and the righteous have also not given a decision regarding it, then exercise ijtihad according to your own opinion, and he should not say: I fear, I fear. Surely, the lawful is clear and the unlawful is clear, and between them are some doubtful matters, so leave that which puts you in doubt and choose that which is certain.