What is the ruling if a person who shortened their prayer only remembers after standing up to resume the missed portion?

Primary text

If a person forgets a part of the prayer, salams prematurely, and then only remembers after rising up, they must sit down in order to stand up again to perform the missed portion, as the initial sitting intended for the completion of the prayer is obligatory. The required action is to complete what was missed with the intention for that portion, then complete the remainder of the required sequence. This principle is derived from the narration of Abu Hurairah concerning the Prophet's prayer where he paused, resting his hand on a piece of wood.