Tertullian

“ For peril has no time for food, nor sackcloth any care for satiety’s refinements. Hunger is ever the attendant of mourning, just as gladness is an accessory of fulness.
Through this attendant of mourning, and (this) hunger, even that sinful state, Nineveh, is freed from the predicted ruin. For repentance for sins had sufficiently commended the fast, keeping it up in a space of three days, starving out even the cattle with which God was not angry. ”

“ Daniel alone, trusting to God, and knowing what would tend to the deserving of God’s favour, requires a space of three days, fasts with his fraternity, and—his prayers thus commended—is instructed throughout as to the order and signification of the dream; quarter is granted to the tyrant’s sophists; God is glorified; Daniel is honoured; destined as he was to receive, even subsequently also, no less a favour of God in the first year, of King Darius, when, after careful and repeated meditation upon the times predicted by Jeremiah, he set his face to God in fasts, and sackcloth, and ashes. ”

“ And thus we have already proceeded to examples, in order that, by its profitable efficacy, we may unfold the powers of this duty which reconciles God, even when angered, to man.
Israel, before their gathering together by Samuel on occasion of the drawing of water at Mizpeh, had sinned; but so immediately do they wash away the sin by a fast, that the peril of battle is dispersed by them simultaneously (with the water on the ground) . ”