Speaking to TheStreet in an interview on Thursday, Boeing CEO Dennis Muilenburg said that his company is “working jointly with NASA and building that first rocket space launch system.” He said it’s the world’s “biggest rocket” and Boeing will begin tests flights on it next year. But, when he was asked whether his company or SpaceX would make it to Mars first, Muilenburg’s answer was clear: “Boeing.”

“I firmly believe that the first person to step foot on Mars will get there on a Boeing rocket,” he said.

Musk, who frequently uses Twitter to share his opinions on a variety of topics, responded to a tweet from Jim Cramer pointing to Muilenburg’s comments. His tweet: “Do it.”

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The war of words comes as SpaceX and Boeing continue work on rockets and a variety of other technologies that will ultimately be required to get people to Mars. Musk has previously said that he believes SpaceX spaceships could begin landing on Mars as soon as 2022. Boeing hasn’t offered any concrete dates for when it could land on Mars.

If Musk’s “Do it” comment sounds familiar, it’s because it’s the same thing he said late last year when Muilenburg said that he believed a Boeing rocket would carry the first person to Mars.

In a tweet response at the time, Musk again said, “Do it.”