Musk told Twitter employees that anyone who had not clicked on a link confirming "you want to be part of the new Twitter" by Thursday evening New York time would be considered to have quit.
Three sources who had received the message at Twitter confirmed its content.
The latest move to restructure the company comes after Twitter slashed half of its workforce earlier this month as Musk took control of the social media company.
Elon Musk photo and Twitter logo are seen through magnifier in this illustration taken November 4, 2022.
He also told employees that if "you do not show up at the office, resignation accepted," according to a transcript of a meeting reported by Verge.