Musnad Ahmad 30:3284

Hajjaj narrated to us, Shaiban informed us, he said: Mansur narrated to us, from Sa'd ibn Ibrahim, from 'Umar ibn Abi Salamah, from his father, from Abu Hurairah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Disputation regarding the Qur'an is disbelief."