Shares of Twitter were down 6 percent at $34.58 in extended trading. That is 36 percent below he $54.20 per share Musk agreed to buy Twitter for in April.
Twitter's shares surged after Musk took a stake in the company in early April, shielding it from a deep stock market sell-off that slammed other social media platforms.
But after he agreed on April 25 to buy Twitter, the stock within a matter of days began to fall as investors speculated Musk might walk away from the deal. With its tumble after the bell on Friday, Twitter was trading at its lowest since March.
The announcement is another twist in a will-he-won't-he saga after Musk clinched the deal to purchase Twitter in April but then put the buyout on hold until the social media company proved that spam bots account for less than 5 percent of its total users.
The contract calls for Musk to pay Twitter a $1 billion break-up if he cannot complete the deal for reasons such as the acquisition financing falling through or regulators blocking the deal. The break-up fee would not be applicable, however, if Musk terminates the deal on his own.
Musk's decision is likely to result in a long protracted legal tussle between the billionaire and the 16-year-old San Francisco-based company.
Daniel Ives, an analyst at Wedbush, said Musk's filing was bad news for Twitter.
"This is a disaster scenario for Twitter and its Board as now the company will battle Musk in an elongated court battle to recoup the deal and/or the breakup fee of $1 billion at a minimum," he wrote in a note to clients.
