John Cassian, The Institutes III.10: How it was brought about that they fast on the Sabbath in the city

But some people in some countries of the West, and especially in the city, not knowing the reason of this indulgence, think that a dispensation from fasting ought certainly not to be allowed on the Sabbath, because they say that on this day the Apostle Peter fasted before his encounter with Simon. But from this it is quite clear that he did this not in accordance with a canonical rule, but rather through the needs of his impending struggle. Since there, too, for the same purpose, Peter seems to have imposed on his disciples not a general but a special fast, which he certainly would not have done if he had known that it was wont to be observed by canonical rule: just as he would surely have been ready to appoint it even on Sunday, if the occasion of his struggle had fallen upon it: but no canonical rule of fasting would have been made general from this, because it was no general observance that led to it, but a matter of necessity, which forced it to be observed on a single occasion.