What is the covenant of Subh?

According to a hadith, "[whoever prays the Subh then he is under the protection of Allah's covenant, so do not be treacherous with Allah in his covenant.](http://www.sunnah.com/tirmidhi/2#74) " Evidence exists of many different covenants between God and mankind, each with their own terms and conditions: **What exactly is the nature of this particular covenant, and when was it covenanted?**