The deal is expected to be about half cash and half stock, the sources said, and will price Slack at a premium. Salesforce is set to report quarterly earnings on Tuesday. Slack's market cap was more than $24 billion as of Monday morning.
Shares of Slack rose about 4% Monday on the news. Salesforce shares were down more than 3%.
Salesforce has made several large acquisitions in recent years, including its $6.5 billion deal for MuleSoft in 2018 and its $15.3 billion deal with data visualization company Tableau.
It could also make Slack a more formidable competitor to Microsoft's Teams communication product. Slack filed a complaint against Microsoft with the European Commission this summer, alleging it abused its dominant market position to eliminate competition for Teams. Microsoft has said Teams has benefited during the pandemic from videoconferencing capabilities that Slack lacks.