Elon Musk has some advice for his billionaire rival Jeff Bezos: work harder.

In a recent interview with the Financial Times, the Tesla and SpaceX CEO spoke candidly about his ongoing space race against the Amazon founder. Musk, who was just named FT’s and Time’s Person of the Year, referred to Bezos as a “friend” with “reasonably good engineering aptitude,” but wasn’t putting enough time and energy into his  aerospace company Blue Origin.

“[Bezos] does take himself a bit too seriously,” Musk said in the interview published Wednesday. “In some ways, I’m trying to goad him into spending more time at Blue Origin so they make more progress. As a friend of mine says, he should spend more time at Blue Origin and less time in the hot tub [...] But he does not seem to be willing to spend mental energy getting into the details of engineering. The devil’s in the details.”

Musk went on to say he spends 80 to 90 hours a week working on Tesla and SpaceX projects. Back in September, a source told CNBC that Bezos only dedicates two days a week—Tuesday and Wednesday—to Blue Origin.

Musk’s comments came more than a month after Blue Origin lost its legal battle against NASA. Bezos’ company sued the agency over its $2.9 billion contract with SpaceX, claiming it had “moved the goal post” last minute to ensure SpaceX got the deal over Blue Origin.

“NASA has executed a flawed acquisition for the Human Landing System program and moved the goalposts at the last minute,” Blue Origin said in a statement to People. “In NASA’s own words, it has made a ‘high risk’ selection. Their decision eliminates opportunities for competition, significantly narrows the supply base, and not only delays, but also endangers America’s return to the Moon.”