"And Ephron was sitting" (Genesis 23:10). [The word] "sitting" (yoshev) is written defective, lacking a vav. At that hour they advanced him to be king. When they heard Abraham, who says, "and entreat for me to Ephron the son of Zohar" (Genesis 23:8), [they did this] so that one as great as Abraham should not be purchasing from a lowly man. And from where did Abraham know of the cave? From the ox that ran and entered there.
Why Ephron Was Made King and How Abraham Found the Cave
Midrash Aggadah, Genesis 23:10
ועפרון ישב. ישב חסר וי"ו, אותה שעה הקדימוהו למלך, כששמעו לאברהם הוא אומר ופגעו לי בעפרון בן צחר, שלא יהא גדול כאברהם קונה מאיש קטן. ומנין היה יודע אברהם למערה, מן הבקר שרץ ונכנס לשם: