How is impurity washed from the inside of a vessel?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
If the object being washed is the interior of a vessel, pouring water into it does not count as a washing until that water is poured out, as this is the customary method of cleaning. However, if the vessel holds two qullahs (a large volume) or more, filling it might be considered equivalent to the washings because the water rotates, allowing new parts of the water to flow over the surfaces previously in contact with impurity, analogous to running water passing over it. Ibn Aqil stated that washing the inside requires pouring the water out.
Supporting text
It is debated whether filling a large vessel (two qullahs or more) suffices as a washing without emptying the water, due to the rotation of water inside, but Ibn Aqil maintains that it must be emptied.