What are the sources, whether oral or written, that prove the Assumption of the Virgin Mary’s body?
The Assumption of St. Mary can be found in the New Testament Apocrypha in the Ante Nicene fathers' series:
Although it is apocrypha, we do accept from it whatever meets the tradition of the church and agrees with the Holy Scripture teachings. We have derived our knowledge of the mystery from Apostolic Tradition.
Also another known literary reference to the Assumption is found in the Greek work De Obitu S. Dominae. The belief in the corporeal assumption of St. Mary is founded on genuine writings in the East; it is mentioned in the sermons of St. Andrew of Crete, St. John Damascene, St. Modestus of Jerusalem and others. In the West, St. Gregory of Tours (De gloria mart., I, iv) mentions it first. St. John of Damascus (P. G., I, 96) thus formulates the tradition of the Church of Jerusalem:
St. Juvenal, Bishop of Jerusalem, at the Council of Chalcedon (451), made known to the Emperor Marcian and Pulcheria, who wished to possess the body of the Mother of God, that Mary had died in the presence of all the Apostles, but that her tomb, when opened, upon the request of St. Thomas, was found empty; wherefrom the Apostles concluded that the body was taken up to heaven.
1. St. John Damascene, Encomium in Dormitionem Dei Genetricis Semperque Virginis Mariae, Hom. II, n. 14; cf. also ibid, n. 3
2. The Encomium in Dormitionem Sanctissimae Dominae Nostrate Deiparae Semperque Virginis Mariae, attributed to St. Modestus of Jerusalem, n. 14.