From Sahih Muslim (Hadith 1662):
Urwa reported 'A'isha to be sayidg:I have never seen the Messenger of Allah (ﷺ) observing the supererogatory prayer of the forenoon, but I observed it. And if the Messenger of Allah (ﷺ) 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
Reference: Muslim 1662