Commentary for Berakhot 104:13

במאי קמיפלגי ב"ש סברי אסור להשתמש בשלחן שני גזרה משום אוכלי תרומה

What is the point of difference between them? Bet Shammai hold that it is forbidden to use a table [unclean in] the second degree through fear of those who are eating Terumah;

Abraham Cohen Footnotes to the English Translation of Masechet Berakhot