Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) was asked about semen that gets onto clothes, and he (peace and blessings be upon him) said: "Semen is like spittle or mucus; it is enough for you to wipe it off with a piece of cloth or with idhkhir grass."
Waki' narrated it from Ibn Abi Layla as mawquf (the words of Ibn Abbas), and that is the correct version.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4176