אַל יִשְׁתֶּה אָדָם בַּכּוֹס זֶה וְיִתֵּן עֵינָיו בַּכּוֹס אַחֵר

ביטוי (אנגלית)

A person should not drink from this cup and cast his eyes on another cup

הגדרה (אנגלית)

By extension: One should not think of another partner while with a particular spouse. A proverb against betraying one’s spouse, even in thought.

("And you shall not go astray after your hearts and after your eyes" (Numbers 15:39) – from here Rabbi said: "A person should not drink from this cup and cast his eyes on another cup" – Nedarim 20b. Likewise: "For the eye sees and the heart devises iniquitous thoughts… whoever gazes at women will ultimately come to sin, for it may happen that his children will be illegitimate from his own wife when he drinks from this cup but casts his eyes on another" – Pele Yoetz, letter Kaf, Kelilat Yofi)