How should one proceed if they need one of the containers for drinking while water is mixed?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
If one requires one of the containers for drinking, one must exercise due diligence (taharri) to determine which one is the pure water. One must then perform Wudu using the water deemed pure and perform tayammum (dry ablution) along with it to achieve certainty regarding purification.
Supporting text
This situation differs from mixing pure water with impure water because impure water certainly contaminates the limbs, and there is no security that the second water used might be the impure one, leaving the limbs impure and invalidating the prayer.