What is the earliest manuscript of the Gloria in excelsis Deo (Greater Doxology) hymn?
is found first [in Greek] in St. Athanasius' treatise: de Virginitate1 as part of morning prayer (with Ps. lxii, and the Benedicite) and in the [Greek Old and New Testament manuscript, the] Codex alexandrinus (Vth cent.).2
1. C. 20 (P.G. xxviii, 275). The authenticity of this work, long disputed, now seems more generally admitted. See Eichhorn : Athanasii de vita ascctica testimonia (Halle, 1886) pp. 27 seq., and especially von der Goltz in Texte und Untersuchungen, N.S. xiv, 2 a.
2. As an appendix to the psalms at the end.