Commento su Giona 3:5

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.

believed: because the sailors were in the city and testified about Jonah that they had cast him into the sea, and the rest of his story as it happened. This is why they believed his prophecy and repented completely.

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 Jonah. And so they believed him without him having to provide any signs or wonders. 

They proclaimed a fast: even before the king's warning, they repented on their own, they fasted, and they wore sackcloth.