וַֽיַּאֲמִ֛ינוּ אַנְשֵׁ֥י נִֽינְוֵ֖ה בֵּֽאלֹהִ֑ים וַיִּקְרְאוּ־צוֹם֙ וַיִּלְבְּשׁ֣וּ שַׂקִּ֔ים מִגְּדוֹלָ֖ם וְעַד־קְטַנָּֽם׃
And the people of Nineveh believed God; and they proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them.
Read Jewish philosophy on Jonah 3:5: Rambam's Guide for the Perplexed, Saadia Gaon, and classical Jewish philosophical sources, free online.