Uṣūl al-Ḥadīth (اصولِ حدیث) is the study of rules through which the condition of the sanad and matn is known.
In simpler words, these rules guide an investigation into:
• What is known about the people who transmitted a Hadith and their sequence?
• What is known about the wording that was transmitted?
Here, condition means the relevant matters that help establish whether a Hadith is to be accepted or not.
This field considers both sanad and matn.