Reference for Bava Batra 43:2
אמר רב נחמן בר יצחק ומודה רב הונא בריה דרב יהושע ברוכלין המחזירין בעיירות דלא מצי מעכב דאמר מר עזרא תקן להן לישראל שיהו רוכלין מחזירין בעיירות כדי שיהו תכשיטין מצויין לבנות ישראל
R. Nahman b. Isaac said: R. Huna the son of R. Joshuah also agrees that itinerant spice-sellers cannot prevent one another from going to any given town, because, as a Master has stated, Ezra made a rule for Israel that spice-sellers should go about from town to town so that the daughters of Israel should be able to obtain finery. This, however, only means that they are at liberty to go from house to house [in the strange town], but not to settle there. If, however, the seller is a student, he may settle also, a precedent having been set by Raba in allowing R. Josiah and R. Obadiah to settle, in despite of the rule. The reason he gave was that, as they were Rabbis, they would be disturbed in their studies [if they had to return to their own town].