There’s a Catholic Answers article called Mary Full of Grace that collects early testimonies to Mary’s Immaculate Conception and Assumption. The earliest explicit quote about her Assumption in that article appears to be this one:
300 A.D. - Pseudo-Melito - “If therefore it might come to pass by the power of your grace, it has appeared right to us your servants that, as you, having overcome death, do reign in glory, so you should raise up the body of your Mother and take her with you, rejoicing, into heaven. Then said the Savior [Jesus]: ‘Be it done according to your will.’” (The Passing of the Virgin 16:2–17 [A.D. 300])
That is a very early testimony to Mary’s Assumption. In my opinion, there are also several Earlier examples where the Church Fathers teach or imply the Assumption, examples that are not covered in that Catholic Answers article. For example, St. Hippolytus of Rome and St. Gregory the Wonderworker both lived before the 300s and called Mary “imperishable”:
~235 A.D. - St. Hippolytus of Rome - “The Lord was without sin, made of imperishable wood, as regards His humanity; that is, of the virgin and the Holy Spirit inwardly, and outwardly of the word of God, like an ark overlaid with purest gold.” (Commentary on Psalm 22, as quoted in Haffner, P. The Mystery of Mary. Gracewing Publishers, p. 77)
~262 A.D. - St. Gregory the Wonderworker - “A bulwark of imperishable life hath the Holy Virgin become unto us, and a fountain of light to those who have faith in Christ; a sunrise of the reasonable light is she found to be.” (Homily on the Mother of God)
The Liber Requiei Mariae, or Book of Mary’s Repose, is also earlier, from roughly the 200s A.D.
~200s A.D. - “And our Lord said to them: ‘Let them bring the body of Mary into the clouds.’ … And when they arrived together in Paradise, they placed the body of Mary beside the tree of life. And they brought her soul and placed it upon her body. And our Lord dismissed his angels to their places.” (Liber Requiei Mariae 89, as it appears in Shoemaker, Ancient Traditions of Mary’s Dormition and Assumption. Oxford: Oxford University Press, 2002. p. 163-164)
And there is the implicit testimony of other Church Fathers consider. The early Church Fathers regularly said things that I think are implicit testimony to the Assumption. I’ve collected some examples at this link:
Mary’s Assumption in the Doctrine of the Pre-Nicene Church
I hope that helps. God bless!