Musnad Ahmad 27:474

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Ayyub, from Nafi', from Ibn 'Umar, who said: The Messenger of Allah (ﷺ) said: "The likeness of the Quran, when its companion commits himself to it and recites it by night and by day, is like a man who has camels; if he ties them, he keeps them, and if he releases their ties, they go away. Likewise is the companion of the Quran."