Dates and documentation for "ancient" please.
Over 1600 years, OR, maybe more. There is a writing from the late 2nd century called "The Book of the Passing of the Most Holy Virgin, the Mother of God" attributed to Bishop Melito of Sardis. Once we get to the 4th and 5th centuries the belief is all over Eastern Christendom. In Christianity, will you not allow that that is "ancient"? What this means, of course, is that the bishops who determined the canon of scripture in the 4th century were likely conversant with this traditional belief. If you want 2000 years, I cannot give you that.
Juniper Carol explicitly states that the Transitus literature is a complete fabrication which should be rejected by any serious historian:
"The account of Pseudo-Melito, like the rest of the Transitus literature, is admittedly valueless as history, as an historical report of Mary’s death and corporeal assumption; under that aspect the historian is justified in dismissing it with a critical distaste" (Juniper Carol, O.F.M. ed., Mariology, Vol. l (Milwaukee: Bruce, 1957), p. 150).
History proves that when the Transitus teaching originated the Church regarded it as heresy. In 494 to 496 A.D. Pope Gelasius issued a decree entitled Decretum de Libris Canonicis Ecclesiasticis et Apocryphis.
In the list of apocryphal writings which are to be rejected Gelasius signifies the following work: Liber qui apellatur Transitus, id est Assumptio Sanctae Mariae, Apocryphus (Pope Gelasius 1, Epistle 42, Migne Series, M.P.L. vol. 59, Col. 162). This specifically means the Transitus writing of the assumption of Mary. At the end of the decree he states that this and all the other listed literature is heretical and that their authors and teachings and all who adhere to them are condemned and placed under eternal anathema which is indissoluble. And he places the Transitus literature in the same category as the heretics and writings of Arius, Simon Magus, Marcion, Apollinaris, Valentinus and Pelagius.
Pope Gelasius explicitly condemns the authors as well as their writings and the teachings which they promote and all who follow them. And significantly, this entire decree and its condemnation was reaffirmed by Pope Hormisdas in the sixth century around A.D. 520.
Prior to the seventh and eighth centuries there is complete patristic silence on the doctrine of the Assumption. But gradually, through the influence of numerous forgeries which were believed to be genuine, coupled with the misguided enthusiasm of popular devotion, the doctrine gained a foothold in the Church. The Dictionary of Christian Antiquities gives the following history of the doctrine:
In the 3rd of 4th century there was composed a book, embodying the Gnostic and Collyridian traditions as to the death of Mary, called De Transitu Virginis Mariae Liber. This book exists still and may be found in the Bibliotheca Patrum Maxima (tom. ii. pt. ii. p. 212)....The Liber Transitu Mariae contains already the whole of the story of the Assumption. But down to the end of the 5th century this story was regarded by the Church as a Gnostic or Collyridian fable, and the Liber de Transitu was condemned as heretical by the Decretum de Libris Canonicis Ecclesiasticus et Apocryphis, attributed to pope Gelasius, A.D. 494. How then did it pass across the borders and establish itself within the church, so as to have a festival appointed to commemorate it? In the following manner:
In the sixth century a great change passed over the sentiments and the theology of the church in reference to the Theotokos—an unintended but very noticeable result of the Nestorian controversies, which in maintaining the true doctrine of the Incarnation incidentally gave strong impulse to what became the worship of Mary.
http://www.christiantruth.com/articles/assumption.html