How should clothing be treated if semen stains it but the exact location is hidden?
Chapter on Praying with Impurity and Other Matters
Al-Mughni
Book of Prayer
Primary text
If the location of the semen is hidden, the garment should be rubbed entirely if impurity is ruled, or it is recommended to rub it if purity is ruled. If one prays in it without rubbing, the prayer suffices, according to Al-Shafi'i and others who hold semen to be pure. If purity is ruled, no such action is obligatory, but recommended, similar to when the stain is known.
Supporting text
Ibn Abbas stated the garment should be entirely sprinkled with water. Ibn Umar, Abu Hurayra, and Al-Hasan stated the entire garment must be washed. The argument against sprinkling is that it is ineffective when the location is known, so it is also ineffective when hidden.