Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1727
Abu Musa al-Ash'ari reported Allah's Apostle (may peace be upon him) as saying: Keep refreshing your knowledge of the Qur'an, for I swear by Him in Whose Hand is the life of Mahammad that it is more liable to escape than camels which are hobbled.