There isn't a direct quote or conversation that matches up with it (i.e., Obi-Wan saying something like "Anakin, there is still good in you. You can turn back.") But then, there doesn't need to be. A lot of us got attached to certain specifics with the OT that were not provided, but merely hinted at. Many of us reasonably assumed that "Obi-Wan once thought as you do" is hinting at some events unseen wherein Obi-Wan tries to turn Vader back to the good side. However, if you view the SW films in numeric order, you'll see that the line makes just as much sense without that assumption. How? Vader doesn't specify himself in that statement. The statement can be read more generally, that Obi-Wan once believed that someone who has turned to the dark side could be turned back. Think about it. It is all a matter of dueling assumptions. Someone watching the films in numeric order will have a different assumption about Vader's comment than someone watching the films in release order. From numeric order, one would assume that Vader is referring to some off-screen discussion that Obi-Wan and Anakin must have had at some point where Obi-Wan expresses a belief that someone turned to the dark side could be saved. It makes sense. Obi-Wan taught Anakin a great deal, must have expressed a lot of his own opinions and knowledge. At some point, Obi-Wan told Anakin that but not in the context of trying to turn Vader back as a lot of us assumed.