When impurity is not removed from clothing merely by scraping without washing, and it has become clear and established from the Prophet (peace be upon him) scraping off semen from a garment after it had dried and then praying in it, that semen is not impure.
It is narrated from Sayyidah Aisha (may Allah be pleased with her) that she used to scrape off semen from the garment of the Messenger of Allah (peace and blessings be upon him) when it was dry. Some narrators have reported the hadith in brief, and some have also mentioned the incident of a guest staying with Sayyidah Aisha (may Allah be pleased with her) and her washing the blanket of the Prophet (peace and blessings be upon him). And her statement that: "I have seen myself scraping semen from the garment of the Messenger of Allah (peace and blessings be upon him)."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب تطهير الثياب بالغسل من الأنجاس / 288