The two men—who have been friends for three decades—both have run hugely successful companies that ended up making them a fortune, and both have pledged to give most of their money away within their respective lifetimes.

Gates, whose $115 billion fortune makes him the fourth richest person in the world, credits Buffett for teaching him one of the most valuable lessons he’s ever learned.

The Microsoft co-founder recently sat down with British Prime Minister Rishi Sunak to answer questions generated by A.I. chatbot ChatGPT.

The bot asked both men to share “the most important piece of advice” they’d ever received—and how it had impacted their lives and careers.

“I’ve had great advice,” Gates responded. “Warren Buffett talked about [how] in the end it’s how friends really think of you and how strong those friendships are [that matters]. That was great advice.”

He went on to discuss how, at the beginning of his career, he had an “overly intense” mindset that may have held him back.

“I didn’t believe in weekends, I didn’t believe in vacation, it meant that there were a lot of people who probably could’ve helped me that just didn’t fit in because I had this very narrow view of working style,” he said.

“For the small early Microsoft group, that was okay, but then as we got bigger I had to realize… you’ve got to think about this. I was very intense on myself and I tried to apply that to other people.”

Gates has previously spoken publicly about the lessons he’s learned with 92-year-old Buffett, who is the largest shareholder in Berkshire Hathaway and one of the most successful investors of all time.

The pair became friends after meeting in the summer of 1991.

In a 2016 blog post, Gates said that “everyone should have a friend as thoughtful and kind as Warren Buffett,” adding that he is “constantly learning things from him.”

“At my office I have just two numbers on speed dial: my home and Warren,” he wrote at the time. “If Warren has time for a call, it’s the highlight of my week. When faced with a challenge, we often ask ourselves, ‘What would Warren do?’ It usually leads us to the best answer.”

In the same blog post, Gates also referenced Buffett’s focus on the importance of maintaining relationships.

“Even though he keeps up a hectic schedule, Warren finds time to nurture friendships like few other people I know,” he said.

“He picks up the phone and calls to say hello. He regularly sends articles he’s read in the mail that he thinks [ex-wife] Melinda or I will find interesting. I’ve learned many things from Warren over the last 25 years, but maybe the most important thing is what friendship is all about. It’s about being the kind of friend you wish you had yourself.”