Reference for Bekhorot 4:56

אלא תנא מכירה והוא הדין לשותפות ה"נ

Has not the father of Samuel said: One must not form a partnership with a heathen lest he [the heathen] will be bound to take an oath to him and he will swear in the name of his idol and the Torah says: [And make no mention of the name of other gods.] neither let it be heard out of thy mouth?<span class="x" onmousemove="('comment',' Ex. XXIII, 23. 'Out of thy mouth', caused by thy mouth, i.e., when you are responsible for the heathen's oath, which shows that it is not allowed.');">³⁴</span>

Jastrow