"Let now a little water be taken" (Genesis 18:4): [by means of a messenger. So too] the Holy One, blessed be He, gave them water by means of a messenger, as it is said, "and you shall strike the rock," etc. (Exodus 17:6). The bread, which he gave by his own hand — the Holy One, blessed be He, likewise gave by His own hand, as it is said, "when the LORD gives you in the evening flesh to eat, and bread," etc. (ibid. 16:8).
Why God Repaid Abraham's Water and Bread Measure for Measure
Midrash Aggadah, Genesis 18:4
יוקח נא מעט מים. [על ידי שליח אף] הקב"ה נתן להם מים על ידי שליח, שנאמר והכית בצור וגו' (שמות יז ו) לחם שנתן על ידי עצמו, אף הקב"ה נתן ע"י עצמו, שנאמר בתת ה' לכם בערב בשר לאכול ולחם וגו' (שם טז ח):