Musannaf Ibn Abi Shayba 1:897

Abu Bakr narrated to us, he said: 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 should wash it. And if he does not see a trace in it, he should sprinkle it with water.