The [Gemmorah in Moed Katan 17][1] says:

>א"ר יוחנן... **אם דומה הרב למלאך ה' יבקשו תורה מפיו**  
ואם לאו אל יבקשו תורה מפיו

>Rabbi Yoḥanan said as follows: ... If the teacher is similar to an angel [malakh] of the Lord, then seek Torah from his mouth, etc

How does (should) an angel of Hashem look/behave that a Rabbi must resemble him?


  [1]: https://www.sefaria.org.il/Moed_Katan.17a.5?lang=bi&with=Rashi&lang2=he