Musnad Ahmad 8:82

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from 'Amir ibn Sa'd ibn Abi Waqqas, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, among the greatest of the Muslims in sin regarding the Muslims is a man who asked about a thing and investigated it, until a prohibition was revealed regarding that thing because of his questioning."