Don't waste water, even at a river
لَا تُسْرِفْ فِي الْمَاءِ وَإِنْ كُنْتَ عَلَى نَهْرٍ جَارٍ
La tusrif fi-l-ma'i wa in kunta 'ala naharin jar.
"Do not waste water, even if you are at a flowing river."
Narrated by Abdullah ibn 'Amr. Sunan Ibn Majah 425 (attribution debated; commonly cited)
A short reflection
Consumption is a character trait, not a supply question. Even abundance is no excuse to be careless.