Musnad Abu Ya'la al-Mawsili 6:73

Abu Khaythama narrated to us, Sufyan ibn 'Uyayna narrated to us, from al-Zuhri, from 'Amir ibn Sa'd, from his father, attributing it to the Prophet (may Allah bless him and grant him peace), who said: "The greatest of the Muslims in crime among the Muslims is one who asks about a matter that was not forbidden to the people, then it becomes forbidden to the people because of his questioning."