Musnad Ahmad 30:3072

Waki' and 'Abd al-Rahman narrated to us, from Sufyan, from Sa'd ibn Ibrahim, from 'Umar ibn Abi Salamah, from his father, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Disputation regarding the Qur'an is disbelief."