Sahih Muslim
The Book of Knowledge (Kitab Al-Ilm)
Book Number: 034 / Hadith Number: 6444
Jundub b. 'Abdullah al-Bajali reported Allah's Messenger (may peace be upon him) as saying: Recite the Qur'an as long as your hearts agree to do so, and when you feel variance between them (between your hearts and tongues), then get up (and leave its recital for the time being).