Elon Musk has hit out at a group of Tesla investors who want the automaker to shakeup its board of directors by suggesting that they should buy shares in Ford.

The outspoken’s CEO’s rhetoric comes shortly after Tesla’s value exceeded both that of Ford and General Motors. It has since slipped below that of GM.

A letter from five investment groups on Monday, including Hermes Equity Ownership Services, the California State Teachers Retirement System and CtW Ivnest Group, says Tesla should have its directors re-elected annually and add two new independent directors without ties to Elon Musk.

“We expect that as companies make the transition to publicly-traded status, the governance structures and practices in place at the time of the IPO will evolve to align with the company’s changing strategy. However, Tesla’s seven-member board is largely unchanged from its pre-IPO days,” the letter read.

In response, Musk took to Twitter to say “This investor group should buy Ford stock. Their governance is amazing…,” referencing how the Ford family controls the 113-year-old company.

Musk later announced that he will appoint more independent directors but said it wasn’t because of the investor group.

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