Elon Musk has become the ‘biggest stakeholder’ in Twitter.

Elon Musk, who also owns Tesla and SpaceX, has become one of Twitter’s biggest shareholders, just a week after saying on the platform that he was “seriously contemplating” launching a new social media platform.

The South African billionaire bought a passive 9.2 percent investment worth $2.9 billion, according to a report released by the US Securities and Exchange Commission (SEC) on Monday.

According to a filing with the US Securities and Exchange Commission, the world’s wealthiest individual, who is worth an estimated $273 billion, bought over 73.5 million shares. It means that Musk owns more than four times the shares of Twitter co-founder Jack Dorsey, who owns 2.25 percent of the firm.

Before Wall Street started on Monday, Twitter’s price jumped by more than 26%, putting his shares at almost $3.6 billion, garnering him an additional $1.2 billion in three weeks.

Musk’s move comes just days after he said he was “thinking” building a new social media platform and questioned Twitter’s commitment to free speech in a tweet.

Elon Musk has accumulated over 80 million followers on Twitter since joining the site in 2009, and has used the platform to make a variety of announcements, including publicising a Tesla go-private proposal that landed him in regulatory hot water.