וַֽיַּאֲמִ֛ינוּ אַנְשֵׁ֥י נִֽינְוֵ֖ה בֵּֽאלֹהִ֑ים וַיִּקְרְאוּ־צוֹם֙ וַיִּלְבְּשׁ֣וּ שַׂקִּ֔ים מִגְּדוֹלָ֖ם וְעַד־קְטַנָּֽם׃
니느웨 백성이 하나님을 믿고 금식을 선포하고 무론 대소하고 굵은 베를 입은지라
Abarbanel
The people of Nineveh believed God: that is, that there was a purpose to His word and His decree. Rabbi Avraham ibn Ezra wrote in the name of Rabbi Yeshuah that the sailors went to Nineveh and told them about Yonah. And so they believed him without him having to provide any signs or wonders.
Ibn Ezra
"And they believed"- [this case is] like [it says in the verse]: "and the people believed" (Shemot 4:31).
Malbim
The people of Nineveh believed God: the novelty was that they believed right away and didn't ask for signs or wonders. They immediately began to pray, wearing sackcloth for fasting and submission. The people were not stirred to repentance, because it did not occur to them that they were sinning, because Jonah didn't say so.