Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated that the Messenger of Allah (peace and blessings be upon him) was asked about semen that gets on a garment, and he (peace and blessings be upon him) said: "It is like saliva or nasal mucus; wipe it off with a piece of cloth or with idhkhir grass."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11273