Siman 578

The Rambam wrote in the first chapter, law 9: Just as the community fasts over its distress, so too an individual fasts over his distress. How so? If he had a sick person, or was wandering in the desert, or was imprisoned in a beit ha-asurim—he should fast for it and seek mercy in his prayer, and he says "Anenu...". And he shall not fast on Shabbatot, nor on festivals, nor on Rosh Chodesh, nor on Chanukah and Purim. Thus far his words. And even if he began to fast—he stops on these days. And only regarding the community are there days that if they began they do not stop, as I have written in siman 572, see there, but not regarding an individual (Maggid Mishneh). And the law if he vowed regarding these days was explained in siman 570, see there.

And know that this law—that an individual is obligated to fast over his distress—was not explained in the Shas in any place. Only the Rambam stated this from his own reasoning, for since it is certain that every individual is obligated to pray over his distress, as is written in the psalm of "four who must give thanks": "And they cried out to Hashem...". And consequently he is obligated to fast, for the nullification of distress requires a fast, as is proven from a dream fast. (And perhaps there is a hint to this from this psalm, as it is written: "Hungry and also thirsty...". And even though in its plain meaning it refers to what preceded, that they wandered in the wilderness in a desert way, and therefore they had nothing to eat and nothing to drink, nevertheless it is possible that it also hinted to a fast. And I do not know why the Rambam did not also include "those who go down to the sea." And perhaps because this is the way of the world and he does so by his own will—a fast is not applicable to this; and he only brings a thanksgiving offering afterward, as is written there: "Those who go down to the sea shall thank Hashem for His kindness" and not for a fast. This is not the case with these three, the sick person and the one wandering in the wilderness and the one sitting in prison, for this is not by his will—he properly fasts.)