The Gemmorah in Moed Katan 17 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?