Musannaf Ibn Abi Shayba 35:433

Mahbub al-Qawariri narrated to us, from Malik ibn Habib, from Salim, he said: A man asked him and said: "I had a wet dream in my garment." He said: "Wash it." He said: "It is hidden from me [i.e., I cannot find the spot]." He said: "Sprinkle it with water."