Is moving the ring during Wudu obligatory if water might not reach underneath it?
Chapter on Siwak and the Sunnah of Wudu
Al-Mughni
Book of Purification
Primary text
If there is doubt that water has reached the skin beneath the ring, moving the ring is obligatory to ensure the water reaches that area, as the default assumption is that water has not reached beneath it. The narration from Abu Rafi' states that the Prophet, peace be upon him, moved his ring when performing Wudu.
Supporting text
If the ring is tight and movement is necessary for water to penetrate, movement is required. If the ring is loose enough for water to enter on its own, moving it is not necessary.