ספר בראשית

This brings me to Bereshit Rabbah 49,4 where we are ‎told that it was Avraham’s custom after having served his guests ‎a meal to ask them to give thanks to the Creator of the universe. ‎Why did Avraham not ask them to pronounce a benediction ‎before they commenced eating, as is customary among Jews, and ‎even among many gentiles? The reason may be that before eating ‎these people were pagans, and how could they pronounce a ‎blessing that implied that they had accepted the yoke of serving ‎the one and only Creator? Having eaten, they had performed a ‎commandment through conferring upon their host Avraham the ‎merit of fulfilling the duty to be hospitable; having done so, they ‎themselves had been given a merit by dint of having contributed ‎to Avraham’s merits, so that now when they pronounced a ‎blessing thanking the Lord Who provides food for all of His ‎creatures, they were not mouthing empty phrases. This is why ‎the Midrash continues by relating that if Avraham’s guests would ‎refuse to recite a blessing to the Lord, he would demand that they ‎return to him what they had eaten, etc, etc. The embarrassment ‎caused the guest when he realized that he had partaken of what ‎had not been intended for him, would usually cause the guest to ‎reconsider. Avraham had explained to the guest that unless they ‎provided him with the merit of having performed the ‎commandment of being hospitable, they would in fact have ‎robbed him of something that was his.‎