Kommentar zu Bereschit 21:7

מי מלל לאברהם The word מי is used as an exclamation of praise alluding to the distinguished position of the Being spoken of. The sense therefore is: See, Who it is, and to what extent He keeps his promise! The Holy One, blessed be He, promises and performs!

‘MI’ (WHO) WOULD HAVE SAID UNTO ABRAHAM. The word mi is used as an exclamation of praise and distinction. The sense of the verse is thus: “See Whom it is and to what extent He keeps His promise. He promises and performs!” Thus the language of Rashi.
But we do not find the word mi used in this way in expressions of distinction and honor. Instead, we find it used only in a derogatory sense: ‘Mi’ (Who is) Abimelech, and ‘mi’ (who is) Shechem?³¹⁹Judges 9:28. ‘Mi’ (Who is) the son of Jesse?³²⁰I Samuel 25:10.
The correct interpretation appears to me to be that Sarah said, “G-d hath made laughter for me; and every one that heareth will laugh on account of me,³²¹Verse 6 here. filling his mouth with song and laughter for the wonder that has been done to me, for who among the hearers would have previously said to Abraham that Sarah will suckle children? There is not a person in the world who would have told him this, even merely to console him, for the possibility would never have occurred to anyone.” Onkelos’ rendition is close to this interpretation: “How faithful is He who spoke to Abraham! He has fulfilled His word that Sarah will suckle children.” That is to say, “Everyone that heareth will laugh on account of me” for there is no person who would have maintained his credulity even in the eyes of Abraham if he were to have told him this wonder.

ותאמר: מי מלל לאברהם, everyone knows that it was the One and only G’d Who had foretold Avraham that Sarah would bear a son for him.