Musannaf Ibn Abi Shayba 35:430

Abu al-Ahwas narrated to us, from Simak, from Ikrimah, from Ibn Abbas, who said: If a man becomes junub (ritually impure) in his garment and sees a trace (of semen) in it, he must wash it. If he does not see a trace in it, he must sprinkle it with water.