What is the ruling when semen is found in a garment used only by the observer?

Primary text

Washing (Ghusl) is obligatory upon the person who observes semen in a garment that no one else sleeps in. This ruling is supported by the actions of Umar and Uthman, who performed Ghusl upon finding it in their garments. Furthermore, since no one else could have deposited it, it is assumed to be from the observer. The observer must repeat prayers performed after any sleep taken in that garment unless evidence suggests the semen predates the last sleep, in which case they repeat prayers back to the earliest sleep where contamination is possible.