Komentarz do Rodzaju 18:9

ויאמרו אליו AND THEY SAID UNTO HIM — The letters 'א' י' ו of the word אליו have dots over them (thus distinguishing these letters which form the word meaning “where is he”?). R. Simeon the son of Eleazar said: wherever you find in a particular word or phrase that the letters in ordinary writing are more numerous than those dotted, you should give a special interpretation to those in ordinary writing. Here the dotted letters are more than those in ordinary writing and you, therefore, give an explanation of the dotted letters — that of Sarah also, they asked, איו “where is he (Abraham)”? So we may learn that in his inn a man should enquire of the man (the host) as to his wife’s welfare and of the woman (the hostess) about her husband’s. In Treatise Bava Metzia (87a) it is stated: The ministering angels knew, indeed, where our mother Sarah was but they asked this question in order to call attention to her modesty (retiring disposition) and so to endear her all the more to her husband.) R. José the son of Haninah stated that they enquired where she was in order to send her the wine-cup of blessing (the glass of wine held by him who recites the Grace after meals)

?איה שרה אשתך, even though the angels knew Sarah’s whereabouts they asked, just as G’d did in Genesis 3,9 when He asked Adam about his whereabouts although He knew full well where Adam was. A similar situation is found in Numbers 22,9 when G’d asked Bileam who the men were who had come to spends the night with him. G’d knew perfectly well who they were.

Where is Sarah. Avraham already knew of Yitzchok’s impending conception (see 17:19). Thus the angels were sent specifically to inform Sarah. This was in order that her rejoicing would have a positive influence on the fetus.