What is the ruling regarding reserving water for one's own survival when others are present and thirsty?

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If one finds a thirsty person whose life is feared lost, giving them water is obligatory, and the person obligated must perform tayammum instead of ablution. This is supported by the precedence of human sanctity over ritual purification. Evidence is found in the report concerning a prostitute who quenched her thirst and then watered a panting dog suffering from thirst; God forgave her for this act, implying that a greater reward is due for saving a human being.