The rules and terminology of the science of Hadith.
The Quran and Sunnah are the two foundations by which Allah established the proof against His slaves. And upon these two sources the rulings of belief and action are built, whether by way of affirmation, or negation.
As for the one who seeks to use the Sunnah as evidence, then two things require investigation:
1. That it is established from the prophet, since not everything that is attributed to him is authentic.
2. What the text indicates in terms of its ruling.
So as a result of this first area of investigation, it was necessary to lay down some principles to discern thereby, that which has been attributed to the Prophet and is to be accepted, from that which is to be rejected. As a result of that, the scholars May Allah have mercy on them, have established that and called it Mustalah Al-Hadith.