Of the Erection of Churches in Bethlehem, and on the Mount of Olives
In the same country he discovered other places, venerable as being the localities of two sacred caves: and these also he adorned with lavish magnificence. In the one case, he rendered due honor to that which had been the scene of the first manifestation of our Saviour's divine presence, when he submitted to be born in mortal flesh; while in the case of the second cavern he hallowed the remembrance of his ascension to heaven from the mountain top. And while he thus nobly testified his reverence for these places, he at the same time eternized the memory of his mother, who had been the instrument of conferring so valuable a benefit on mankind.
Source: Life of Constantine (New Advent)