From another website:
Parshas Vayeira: Why Was The Generous Avrohom Frugal When It Came To The Water?
The Meforshim answer, the pasuk says, “Yukach Na” – “Let it be taken” – which Rashi explains to mean – “Al Yidei Shliach” – by means of a messenger. Since it was another person who would be carrying the water, Avraham was sensitive not to make him carry such a heavy load and therefore only offered the guests “a bit of water”. Eventhough he was busy with the Mitzvah of Hachnosas Orchim – Avraham teaches us that one cannot be “Frum on Yenim’s Cheshbon”