Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1548
'Urwa reported 'A'isha to be sayidg: I have never seen the Messenger of Allah (may peace be upon him) observing the supererogatory prayer of the forenoon, but I observed it. And if the Messenger of Allah (way peace be upon him) abandoned any act which he in fact loved to do, it was out of fear that if the people practised it constantly, it might become obligatory for them.