Musnad Ahmad 27:311

Waki’ narrated to us, al-‘Umari narrated to us, from Nafi’, from Ibn ‘Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The likeness of the Quran is like that of hobbled camels; if their owner tends to them, he keeps them, and if he leaves them, they go away."