Musannaf Ibn Abi Shayba 1:850

Hushaym narrated to us, from Abu Hamza, who said: While I was traveling on my riding beast, and I was between sleeping and waking, I felt a desire, and I disapproved of myself. Then there came out of me that which wet my garment, and there was no one there. So I asked Ibn Abbas, and he said: "Wash your private part and whatever [of the fluid] reached you," and he did not command me to perform the ritual bath (ghusl).