Ibn Shihab al-Zuhri (may Allah have mercy on him) said: Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said while on the pulpit: O people, correct opinion was only that of the Messenger of Allah (peace and blessings be upon him), because it was from Allah, the Lord of Might; from us it is only conjecture and affectation.
The Shaykh said: By this he meant, and Allah knows best, opinion that does not resemble a source (asl); and what is narrated from him and others in condemnation of opinion carries this meaning, for it has been narrated to us that most of them exercised ijtihad by opinion where there is no text. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20358