Tafsir of Ad-Dhariyat 51:28

(Fa-awjasa minhum khifah) means he concealed within himself fear of them, when he—peace and blessings be upon him—saw their aversion to his food. He thought that this indicated an evil intention toward him, for the guest’s eating is a sign of safety and a proof that the host and the food have sanctity and protection, while refraining from it is a cause of alienation, necessitating the suspicion of evil.

It is narrated from Ibn Abbas that it occurred to his heart—peace be upon him—that they were angels sent for punishment, so he feared. They said, "Do not fear; (We are messengers of your Lord)."

Regarding Yahya bin Shaddad, Gabriel—peace be upon him—wiped the calf with his wing, and it stood up and walked until it reached its mother; so he knew them and felt safe from them. According to what is narrated from the scholar (Ibn Abbas), this was merely to reassure him—peace be upon him. It is also said that it happened after confirming that they were angels and knowing what he had concealed in his heart, either by Allah the Exalted informing them of it, or His angels—the recording scribes—informing them of it, or by the appearance of its signs upon his noble face, from which they inferred the inner state.

(And they gave him glad tidings), and in Surah As-Saffat it is written "(And We gave him glad tidings)," meaning through them, (of a boy). According to the majority, he is Isaac, son of Sarah, and this is the truth, for there is a textual stipulation that he is the one promised in Surah Hud, and the story is one and the same. Mujahid said: He is Ishmael, son of Hajar, as narrated from him by Ibn Jarir and others, though this is hardly correct.

(Possessing knowledge): Meaning upon his reaching maturity and strength. In this is a glad tiding of his long life. The glad tiding was specifically of a male child, as it is more pleasing to the soul and more delightful. He was described as having "knowledge" because it is the attribute that distinguishes the perfect human being, not physical beauty, strength, or the like. This is according to those other than the majority of people in this time, for with them, knowledge—especially religious knowledge—is a vice that has no equal, and ignorance is a virtue that has no peer. In the intensive form (of the noun) with the omission of the object, there is that which is not hidden of what necessitates joy.

It is narrated from Al-Hasan that "(Possessing knowledge)" means a prophet. The glad tiding occurred after the reassurance, and in that is an indication that averting harm is more important than bringing about benefit. Some mentioned that his knowledge—peace be upon him—that they were angels came from the fact that they gave him tidings of the Unseen.