Musnad Ahmad 27:397

Muhammad ibn Ubayd narrated to us, he said: Ubaydullah 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 only like that of hobbled camels; if their owner attends to them by keeping them hobbled, he retains them, but if he releases their hobbles, they go away."