After acquiring a 9.2% stake in Twitter not too long ago, the world's richest man said he is now willing to buy all of Twitter's shares.

Elon Musk announced on Thursday that he is willing to buy all of Twitter's shares for $54.2 each, according to a filing with the Securities and Exchange Commission (SEC).

"I am offering to buy 100% of Twitter for $54.20 per share in cash, a 54% premium over the day before I began investing in Twitter and a 38% premium over the day before my investment was publicly announced," Musk said in a letter accompanying the filing.

Twitter's shares have already jumped 13% on the news.

Musk added in the filing "My offer is my best and final offer and if it is not accepted, I would need to reconsider my position as a shareholder. Twitter has extraordinary potential. I will unlock it."

This possible deal would place Twitter's market capitalization above $43 billion.

Musk had earlier turned down a seat on the board of directors. It is speculated that being on the board would have capped Musk's ownership stake at %14.9. Turning down a board seat would allow him to accumulate shares beyond 14.9%, and make significant changes to the social media giant.