So about the 5th. cen. is the earliest known for sure.
“Our first Syriac documents come from about the end of the fifth century (”Testamentum Domini,” ed. by Ignatius Rahmani II, Life of Severus of Antioch, sixth century).”
Catholic Encyclopedia
“So about the 5th. cen. is the earliest known for sure.”
BTW, I’d take most anything from the “Catholic Encyclopedia” with several tablespoons of salt, cyc!
The anaphora and what they said are referred to in the Didache which is from about 100 AD. The earliest actual fragments of Euchologia for various Liturgies probably are from the 400s, but again, these are developments from what, for example, +Ignatius of Antioch was doing in the late 1st and very early 2nd centuries, with the anaphora being remarkably consistent even to this day in the East.
You know, there are East Syriac anaphora also, similar to the West Syriac ones. One or more of these are also used by the Maronites and also the Assyrians and Chaldeans who are Oriental Orthodox. There are dozens of anaphora from both the East and West Syrian praxis. It is conceivable that these were translations and variations on some Greek language anaphora used in the first couple of centuries after Christ.