Rashi on Genesis 16:8/רש"י על בראשית ט״ז:ח׳

1אי מזה באת WHENCE CAMEST THOU? — He knew it, but he asked this in order to give her a starting-point so that he might converse with her. The expression אי מזה is to be explained thus: Where is (אי) that place of which thou canst say, “From this (מזה) have I come”.