Surah 51. Az-Zariyat, Ayah 26

 Abraham (ﷺ) did not want his guests to know that he was going to prepare a meal for them so they would not ask him not to.

They seemed unusual strangers, but he said nothing and quietly proceeded to perform the rites of hospitality. He brought a roast fatted calf and placed it before them to eat. But the strangers did not eat (xi. 70). This disconcerted him. According to the laws of hospitality, a stranger under your roof is under your protection, but if he refuses to eat, he refuses your hospitality and keeps himself free from any ties of guest and host. "What were their designs?" thought Abraham, and he felt some distrust. But they were angels and could not eat. They declared themselves, and announced the birth to Abraham of a son endowed with wisdom,-in other words that Abraham was to be the head of a long line of Prophets! (xv. 53).