Reference for Moed Katan 33:6

שמתיה רב יהודה לסוף איחלש רב יהודה אתו רבנן לשיולי ביה ואתא איהו נמי בהדייהו כד חזייה רב יהודה חייך

[It means, that] if the Master is like unto a messenger of the Lord of Hosts, they should seek the law at his mouth; but if [he be] not , they should not seek the law at his mouth'. [Thereupon] Rab Judah pronounced the shammetha on him. In the end Rab Judah became indisposed.

Jastrow