Musannaf Ibn Abi Shayba 22:79

Abu Khalid al-Ahmar narrated to us, from Ubayd Allah, from Nafi’, from Ibn Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The likeness of the Quran is like that of hobbled camels; if their owner keeps them tied, he retains them, and if he leaves them, they go away."