Genesis 18:9

וַיֹּאמְרוּ אֵלָיו אַיֵּה שָׂרָה אִשְׁתֶּךָ וַיֹּאמֶר הִנֵּה בָאֹהֶל

They said to him, "Where is Sarah, your wife?" He replied, "Behold, in the tent."

The three travelers who appeared to Abraham were angels in the guise of humans. Rabbi Joseph B. Soloveitchik says that by “where is Sarah, your wife?” they angels were pointing out that Sarah was Abraham’s equal partner in his great work, without whom nothing he did would be possible. His reply, that she was in the tent, highlighted not only Sarah’s modesty but also that only particularly perceptive people would be aware of the great role she played. Indeed, after Sarah’s death, Abraham ceases to be the central character in the Biblical narrative. (From Family Redeemed, cited in the Mesoras HaRav Chumash.)