The chief executive officer of messaging unit WhatsApp confirmed in a Facebook post Monday afternoon that he was leaving the company. The announcement comes before the final three vesting dates of restricted stock unit awards tied to Facebook’s $22 billion purchase of WhatsApp in 2014.

Koum, 42, got about 24.9 million restricted shares as part of the deal. The stock vests in increments until late 2018, with 1.9 million shares due to vest in mid-May and mid-August, plus a final tranche of 2.1 million set to be issued in November.

The awards are contingent on him still being employed through those dates. He would forfeit 5.8 million shares, worth $997.5 million as of Monday’s close, if he left before May 15, unless his exit is categorized as an involuntary termination or a good-reason resignation, regulatory filings show.

The precise circumstances of his departure aren’t yet clear. Facebook (FB) declined to comment on Koum’s departure date.

The Washington Post reported earlier that Koum is exiting the company after clashing with Facebook over strategy, and that he also plans to leave the board. The other co-founder of WhatsApp, Brian Acton, also left recently — and last month posted the #DeleteFacebook hashtag during the social network’s scandal over user privacy.

Koum’s decision may be made easier thanks to the $10.4 billion fortune he’s already accrued, according to the Bloomberg Billionaires Index. He’s already sold $8 billion worth of Facebook stock since 2015, according to data compiled by Bloomberg.

“It is time for me to move on,” Koum said in the post. “I’m taking some time off to do things I enjoy outside of technology, such as collecting rare air-cooled Porsches, working on my cars and playing ultimate Frisbee.”