Assumption of Moses (or Testimony of Moses)

Nicephorus of Constantinople (mid-800s), who compiled a listing of books used by Christians at that time, included both these books and described the Testament of Moses as having 1100 lines and the Assumption of Moses as having 1400 lines. It is possible but unlikely that Nicephorus confused these two or that our copy is one or the other, although no one is certain which writing it would be.

A Testimony means that one person is passing on instructions to another, in this case, from Moses to Joshua.
The actual assumption of Moses is not included. We only have a partial copy, and the description of the assumption of Moses, if it existed, was in the missing part. Still, early church leader Origen (d. 254) states that the Assumption of Moses was used as a source of information about Michael the archangel and Satan debating over Moses' body in the book of Jude: "And in the first place, in the book of Genesis, the serpent is described as having seduced Eve; regarding whom, in the work entitled "The Ascension of Moses" (a little treatise, of which the Apostle Jude makes mention in his Epistle), the archangel Michael, when disputing with the devil regarding the body of Moses, says that the serpent, being inspired by the devil, was the cause of Adam and Eve's transgression." (Origen, De Principis, III.2.1)

The original may have contained the passage that Jude 9 refers to. Unfortunately, the copy we have is incomplete, the ending is missing, and the part we do have does not have that reference. Therefore, we don't know for certain. The only information we have is the secondary source above from Origen.