Musannaf Ibn Abi Shayba 1:854

Yazid ibn Harun narrated to us, he said: Sa'id ibn Abi 'Aruba informed us, from 'Ali ibn Thabit, from Nafi', from Ibn 'Umar, that he was asked about a man who woke up from his sleep and saw moisture. He said: "If I had found that, I would have performed ghusl (ritual bath) because of it."