From paragraph 44 of Munificentissimus Deus:
...we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.
There is no scriptural evidence to support this dogma, as the Catholic Encyclopedia admits:
The earliest known literary reference to the Assumption is found in the Greek work De Obitu S. Dominae. Catholic faith, however, has always derived our knowledge of the mystery from Apostolic Tradition...
...The belief in the corporeal assumption of Mary is founded on the apocryphal treatise De Obitu S. Dominae, bearing the name of St. John, which belongs however to the fourth or fifth century. It is also found in the book De Transitu Virginis, falsely ascribed to St. Melito of Sardis, and in a spurious letter attributed to St. Denis the Areopagite. If we consult 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 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.