Is the text: "If you see a man clinging to an opinion, supplicate to Allah to cure him, for he has gone astray" a prophetic hadith, a saying of a companion, a saying of a scholar, or a saying of a wise man?

No hadith (definition) with this exact wording has been found, but it is established that fanaticism and adherence to one's whims are causes of misguidance. Following desires was the cause of Bal'am bin Ba'ura's destruction, as mentioned in the words of Allah Almighty: "And recite to them the news of him to whom We gave Our verses, but he slipped away from them; so Satan pursued him, and he was among the deviators. And if We had willed, We could have elevated him thereby, but he adhered to the earth and followed his desire. So his example is like that of the dog: if you chase him, he pants, or if you leave him, he pants. That is the example of the people who denied Our verses. So relate the stories that perhaps they may reflect." (Al-A'raf 7:175-176). And His saying, the Almighty: "But if they do not respond to you, then know that they only follow their [own] desires. And who is more astray than one who follows his desire without guidance from Allah? Indeed, Allah does not guide the wrongdoing people." (Al-Qasas 28:50). It was narrated that Ali (may Allah be pleased with him) said: "The two things I fear most for you are following desires and prolonged hope. For following desires hinders from the truth, and prolonged hope makes one forget the Hereafter."