Answered by Shaykh Amjad Rasheed
Is it permissible to use the water of the masjid for a purpose other than what is meant for? What if the custom of the people to use it for something else is commonly accepted? Or if one only uses a little?
It is not permissible for a person to use the water of a masjid for something other than what it is meant for. So if it is for drinking, its purpose is restricted to that. If it is for purification, it is restricted to that. And if the water is for both purposes, as it usually is, then it can only be used for those two purposes, as [s. the imams] have said. This is regardless of whether that is a little or a lot, as the expressions of the scholars expressly indicate.
– Amjad Rasheed
(Translated by Shazia Ahmad)