question #3 Sahih Muslim 648b, Book 5, Hadith 299 : How can performing prayers at their prescribed times serve as a supererogatory act?

The Messenger of Allah (ﷺ) said to me: O Abu Dharr, you would soon find after me rulers who would make their prayers dead. You should say prayer at its prescribed time. If you say prayer at its prescribed time that would be a supererogatory prayer for you, otherwise you saved your prayer.