I let ChatGPT tell me some proverbs related to "你要教人家一杯水，你自己就要有一桶水的储备" and it tells me that one. Although I have searched on Baidu and Google, I have been unable to find any relevant sources. Therefore, I am curious to know if the saying "先学为后己，后学为先己" exists in Chinese? Or in any other languages? I am Chinese but I don't know any relevant sources.
As others have said, this is not an existing proverb in Chinese. Chatgpt is a very advanced AI by modern standards, but it is not actually capable of properly understanding context or being a reliable source of information. (If interested look into winograd schema and similar concepts, such things Chatgpt cannot pass).
Chatgpt is able to very scientifically analyze languages from a mathematical and computerized way. Sometimes that results in things that are completely wrong, or things like this that are believable. As an example of how chatgpt may have come to this:
The importance of learning for your future is very widespread in Chinese culture, especially for some people, it is genuinely their main way to improve quality of life. So it is reasonable to say that Chatgpt will have many, many sources to analyze on the subject. It also has access to actually existing proverbs in huge quantities.
Analyzing these, it can compare which characters appear together the most, whether next to each other or in the same article. It can analyze the format of existing proverbs, and combine it together. Even text prediction like a keyboard will be used to a sophisticated degree. Thus a realistic but false answer is born.
So once again we confirm that Chatgpt is advanced in many ways, and very fun to play with. It is still far from a reliable source of information. It truly is the language equivalent of AI art, the main theme is maintained but the details are likely to be distorted. AI art may seem unrelated, but the way they function is the same-- one scientifically tries to analyze art and the other language.
As you can see, a picture is worth a thousand words. This AI was given the prompt "cat chinese inkwash painting". It does not understand, and cannot comprehend. It analyzes the individual terms with their search results, and analyzes cat art in general. What it has returned is indeed a cat art-- however the integrity of the request is nowhere to be seen. All modern AI are this level, where the main goal is to create something realistic, with almost no thought into accuracy (since it isn't possible yet).