Reference for Avodah Zarah 34:13

ולא אשגח ביה קם למימר להו הוה כתיבא איגרתא דהוה כתיב מחשיבי דמלכות לשדורי לבי קיסר ושדרוה על ידיה דההוא גברא אתא אליהו פתקיה ארבע מאה פרסי אזל ולא אתא

He, however, disregarded him. He stood up to tell them [that R. Elazar had freed his slave]. There was a written communication from important members of the government that had to be sent to the Emperor and they sent it by that man. [On the road] Elijah came and hurled him a distance of four hundred parasangs. So that he went and did not return.

Jastrow