In the "Star Wars" universe, particularly in the animated series "Star Wars: Rebels," there is a storyline involving the Inquisitors, who are Force-sensitive agents working for the Sith to hunt down Jedi survivors after the Clone Wars. However, there is no specific canon material that details Obi-Wan Kenobi stealing a lightsaber from an Inquisitor after the Clone Wars.
In the "Kenobi" series (2022), which takes place between "Revenge of the Sith" and "A New Hope," Obi-Wan is shown evading the Empire but does not specifically steal a lightsaber from an Inquisitor.
If you were referring to a specific scene in a book, comic, or fan work, it would be helpful to clarify that, as the Star Wars universe is extensive and includes various forms of media.