He conceived fear of them; they said: You don’t fear, and they gave him Glad Tidings of a son having Knowledge.
On seeing that their hands were not stretched to the food tray, Ibrahim feared the guests. Since Ibrahim couldn’t identify that the guests were the Angels, he prepared food for them. See explanation 18: 77-78.
The son having the Knowledge implies Ishaq. The forbearing son about whom Ibrahim was given Glad Tidings in verse 37: 101 is Ismael. It is told in verse 11: 71 that the wife of Ibrahim, Sara, was informed about Ishaq and behind him about Yahqub also. So the first son of Ibrahim was the forbearing son Ismael, born to Haajara, who was to undergo sacrifice; and the second son of Ibrahim was the Knowledge-having Ishaq, who was born to Sara. It is notable that just like Ibrahim was given Glad Tidings about the son Ishaq, Sara was also given the same Glad Tidings. While giving Glad Tidings to Ibrahim about Ishaq, he was also given Glad Tidings about Yahqub, who is the son of Ishaq. From this, it is clear that Ishaq, who has to pour the knowledge, was not commanded to be sacrificed.