Musnad Abu Ya'la al-Mawsili 6:76

Muhammad ibn 'Abbad al-Makki narrated to us, Sufyan ibn 'Uyaynah narrated to us, he said: I memorize it just as I memorize [it]: From al-Zuhri, from 'Amir ibn Sa'd, from his father, he said: The Messenger of Allah, may Allah bless him and grant him peace, said: "The greatest of the Muslims in sin among the Muslims is one who asks about a matter that was not forbidden to the people, then it became forbidden because of his asking."