Travis Kalanick, chief executive officer of Uber, once approached Tesla CEO Elon Musk about working together on self-driving cars, but Musk said no, according to an upcoming book. What’s interesting to note is that a few days later, Musk announced Tesla’s own plan to launch an autonomous car-sharing fleet.

Uber Tesla self-driving Musk Kalanick
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Why Musk refused the offer from Kalanick

According to the book Wild Ride written by Fortune’s Adam Lashinsky, Kalanick called Musk last year to propose a partnership on self-driving cars. The book, which was read by Bloomberg in advance of its release next week, claims that Kalanick pitched Tesla’s CEO the idea of teaming up against Apple.

This discussion between Musk and Kalanick happened after Apple invested $1 billion in Didi Chuxing, Uber’s arch-rival in China at the time. Uber has now given up the battle in China and sold its business to Didi.

The book has Kalanick recalling his words to Musk: “Look man, we should partner.”

In the rest of the call, Musk reportedly tried to convince Kalanick that the idea is “too far out, and it’s not realistic, that I should just stick to what we do best and be focused, or I’m going to f— it all up. That’s when I knew Tesla was competing,” recalled Kalanick.

Kalanick told Lashinsky that the Tesla CEO may have been acting clever (by refusing his offer), but the ride-sharing company later learned from a Tesla representative that self-driving cars were a top priority for the automaker.

Musk may have cleverly refused Kalanick’s offer to work together on self-driving tech, but he did not even attempt to dissuade the Uber CEO from working on autonomous cars, said a person close to Musk, according to Bloomberg.

Both Tesla and Uber working on self-driving tech now

At that time, the automaker was rolling out its semi-autonomous driving system Autopilot to its electric cars. Just days later after brushing off Kalanick’s offer, the Tesla CEO revealed plans to enter the ride-hailing business as possible competition for Uber.

Last year in his “Master Plan Part Deux,” the Tesla CEO wrote, “When true self-driving is approved by regulators, it will mean that you will be able to summon your Tesla from pretty much anywhere.”

Musk added that Tesla EV owners can have the car generate income for them just by adding it to the Tesla shared fleet by tapping a button on the Tesla’s phone app.

Whether or not Kalanick’s proposal played a hand in encouraging Musk to seriously consider car-sharing is not known. But Kalanick did say that he was aware that the automaker was planning to compete when Musk turned his offer down, notes Business Insider.

Uber is now working on its own self-driving technology. Currently, the ride-hailing company is working with Volvo and Daimler AG on self-driving cars. The Uber CEO wanted to buy over 500,000 Tesla EVs if they were fully autonomous by 2020, said Steve Jurvetson, an investor in Tesla and a Silicon Valley venture capitalist.

As of now, there have been no comments from Tesla or Uber on the book.