Responsa על מנחות 85:22

Q. Whence did Maimonides derive the law that one should not sell to a Gentile a talith unless its Zizith are cut off, lest the Gentile gain the confidence of a Jew thereby, and kill him while traveling with him?
A. This law is clearly stated in the Talmud, (Menachot 43a).
This Responsum is addressed to R. Asher.
SOURCES: Pr. 543.