Augustine of Hippo — as excerpted in the Catena Aurea on Luke 4:1-4

“(de Con. Ev. lib. ii. c. 4.) Now that number is a sacrament of our time and labour, in which under Christ’s discipline we contend against the devil, for it signifies our temporal life. For the seasons of the year run in courses of four, but forty contains four tens. Again, those ten are completed by the number one successively advancing up to four. This plainly shews that the fast of forty days, i. e. the humiliation of the soul, the Law and the Prophets have consecrated by Moses and Elias, the Gospel by the fast of our Lord Himself.”