Pope Pius XII's 1 Nov. 1950 Apostolic Constitution defining the dogma of the Assumption of the Blessed Virgin Mother, Munificentissimus Deus, says "that, since ancient times, there have been both in the East and in the West solemn liturgical offices commemorating this privilege." He then mentions the Roman liturgy, Gallican sacramentary, and the Byzantine liturgy of the Dormition and Assumption of the Blessed Virgin Mother.
Pope Pius XII also mentions or quotes St. Anthony of Padua, St. Damascene, St. Albert the Great, St. Thomas Aquinas, St. Bonaventure, St. Robert Bellarmine, St. Alphonsus Liguori, St. Peter Canisius, and Suarez, all of whom supported the Assumption (or at least did not deny it).
St. John Damascene (675-749), Encomium in Dormitionem Dei Genetricis Semperque Virginis Mariae, Hom. II, n. 14; cf. also ibid, n. 3.:
It was fitting that she, who had kept her virginity intact in childbirth, should keep her own body free from all corruption even after death. It was fitting that she, who had carried the Creator as a child at her breast, should dwell in the divine tabernacles. It was fitting that the spouse, whom the Father had taken to himself, should live in the divine mansions. It was fitting that she, who had seen her Son upon the cross and who had thereby received into her heart the sword of sorrow which she had escaped in the act of giving birth to him, should look upon him as he sits with the Father. It was fitting that God's Mother should possess what belongs to her Son, and that she should be honored by every creature as the Mother and as the handmaid of God.
St. Germanus of Constantinople (8th century), In Sanctae Dei Genetricis Dormitionem, Sermo I.:
You are she who, as it is written, appears in beauty, and your virginal body is all holy, all chaste, entirely the dwelling place of God, so that it is henceforth completely exempt from dissolution into dust. Though still human, it is changed into the heavenly life of incorruptibility, truly living and glorious, undamaged and sharing in perfect life.
St. Anthony of Padua (b. 1195), explaining "I will glorify the place of my feet" (Is 61:13.), said (Sermones Dominicales et in Solemnitatibus, In Assumptione S. Mariae Virginis Sermo.):
you have here a clear statement that the Blessed Virgin has been assumed in her body, where was the place of the Lord's feet. Hence it is that the holy Psalmist writes: "Arise, O Lord, into your resting place: you and the ark which you have sanctified."
And that she
has risen up, since on this day the Virgin Mother has been taken up to her heavenly dwelling.
There are more quotes like these in Munificentissimus Deus.
See also this quote and Shoemaker's Ancient Traditions of the Virgin Mary's Dormition and Assumption.