Musannaf Ibn Abi Shayba 1:928

Hussain bin Ali narrated to us, from Ja’far bin Burqan, from Khalid bin Abi ‘Izzah, who said: A man asked Umar bin al-Khattab and said, "I had a wet dream on a rug." He said, "If it is wet, wash it. If it is dry, scrape it. And if it is unclear to you, sprinkle it with water."