Musnad Abu Ya'la al-Mawsili 6:74

The Al-Qawariri narrated to us, Sufyan bin Uyaynah narrated to us, from Al-Zuhri, from Amir bin Sa’d, from his father, from the Prophet (peace and blessings of Allah be upon him), who said: "The greatest of the Muslims in sin is the one who asks about a matter that was not forbidden to the people, then it becomes forbidden because of his questioning."