Marc Benioff, chairman and chief executive officer of Salesforce.com speaks during the grand opening ceremonies for the Salesforce Tower in San Francisco on May 22, 2018.
David Paul Morris | Bloomberg | Getty Images
The deal is expected to be about half cash and half stock, the sources said, and will price Slack at a premium to its current price. Salesforce is set to report quarterly earnings on Tuesday.
Shares of Slack rose about 8% Monday on the news. Salesforce shares fell about 1.5%.
Salesforce has made several large acquisitions in recent years, including its $6.5 billion deal for MuleSoft in 2018 and $15.3 billion deal with data visualization company Tableau.
The deal with Slack could become one of the largest software deals. IBM bought Red Hat for $34 billion in 2019 and Microsoft bought LinkedIn for $27 billion in 2016.
This story is developing. Check back for updates.