According to an SEC filing, Musk has offered to acquire all the shares in Twitter he does not own for $54.20 per share, valuing the company at $43.4 billion. That represents a 38% premium over the closing price on April 1, the last trading day before Musk disclosed that he had become Twitter’s biggest shareholder, and an 18% premium over its closing price Wednesday.

Musk said the cash offer was his “best and final offer,” according to the SEC filing, adding that if it’s not accepted he would have to reconsider his position as a shareholder.

The Tesla CEO sent an offer letter to the company Wednesday night, according to the filing.

“I invested in Twitter as I believe in its potential to be the platform for free speech around the globe, and I believe free speech is a societal imperative for a functioning democracy,” he said in a letter he sent to Twitter. “However, since making my investment I now realize the company will neither thrive nor serve this societal imperative in its current form. Twitter needs to be transformed as a private company.”

Last week, Musk disclosed he had been buying shares of Twitter since late January, and that he had accumulated a 9.2% stake since that time, spending $2.6 billion on the shares he purchased. After that disclosure, he initially accepted an offer of a seat on the board of the company, an agreement that included a cap on his investment in the company to a 14.9% stake.

On Sunday Twitter CEO Parag Agrawal disclosed Musk had decided not to join the board, which removed that limit.