Salesforce Validation Rules are stored in ValidationRule entity. So, we can query the ValidationRule entity to fetch Salesforce Validation Rules using Tooling API.

Query:

SELECT Id, FullName, ValidationName, ErrorMessage
FROM ValidationRule

Single Validation Rule:

/services/data/v48.0/tooling/sobjects/ValidationRule/03d5w0000006lVW

Multiple Validation Rule Tooling API Salesforce

/services/data/v48.0/tooling/query/?q=select+Id,Active,ValidationName+from+validationrule+where+id+in+(’03d5w0000006lVW’,’03d5w0000006lVR’)

Reference Article:

https://developer.salesforce.com/docs/atlas.en-us.api_tooling.meta/api_tooling/tooling_api_objects_validationrule.htm