Rashi on Breishis 18:16
And the men arose from there, and they looked upon Sodom, and Abraham went with them to escort them,
to escort them: Heb. לְשַׁלְּחָם, to escort them. He thought that they were wayfarers
How could Avraham have thought they were just wayfarers, when just a few pesukim earlier in 18:10, one of the angels says
And he said, "I will surely return to you at this time next year, and behold, your wife Sarah will have a son." And Sarah heard from the entrance of the tent, and it was behind him.
From that statement, it would be obvious to Avraham that he was not an ordinary human being, to make such promises.