Chapter IV. How throughout the whole of Egypt and the Thebaid the number of Psalms is fixed at twelve.
Works of St John Cassian (Institutes, Conferences, On the Incarnation) · Saint John Cassian
How throughout the whole of Egypt and the Thebaid the number of Psalms is fixed at twelve.
Cassian 1.15.1
So, as we said, throughout the whole of Egypt and the Thebaid the number of Psalms is fixed at twelve both at Vespers and in the office of Nocturns, in such a way that at the close two lessons follow, one from the Old and the other from the New Testament. And this arrangement, fixed ever so long ago, has continued unbroken to the present day throughout so many ages, in all the monasteries of those districts, because it is said that it was no appointment of man’s invention, but was brought down from heaven to the fathers by the ministry of an angel.
Cassian 1.15.2