Stadler, who was arrested on Monday and is being held at a prison in the southern German city of Augsburg, spoke with investigators in the presence of his lawyer, the source added.
Prosecutors and Stadler's lawyer did not comment on the nature of their discussion. Stadler himself was not reachable for comment.
Audi, the premium brand of carmaker Volkswagen Group, has said Stadler should be presumed innocent until proven otherwise.
Munich prosecutors are investigating him for suspected fraud and false advertising in connection with emissions test cheating by Audi - part of the broader "dieselgate" scandal that has engulfed Volkswagen (VW) since 2015.
They remanded Stadler in custody on Monday to prevent him from obstructing their probe.
Stadler has been under fire ever since Audi admitted to using cheating software in November 2015 - two months after VW - but has enjoyed backing from members of the Porsche and Piech families who control VW and Audi.
"In designing the defeat device, VW engineers borrowed the original concept of the dual-mode, emissions cycle-beating software from Audi," VW said in its plea agreement with U.S. authorities in January 2017, in which the company agreed to pay a $4.3 billion fine to reach a settlement with U.S. regulators.
VW has insisted the development of illegal software, also known as "defeat devices", was the work of low-level employees, and that no management board members were involved.
U.S. prosecutors have challenged this by indicting VW's former CEO Martin Winterkorn. Investigations in Germany are continuing.
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)