Musannaf Ibn Abi Shayba 1:973

Ibn ‘Ulayyah narrated to us, from Ibn ‘Awn, from Anas, from Ibn Sirin, who said: Ibn ‘Abbas said: "Regarding the *mani* (semen), one must perform *ghusl* (full ritual bath) because of it. Regarding the *madhi* (pre-seminal fluid), one must wash his private part because of it and perform *wudu* (ablution). And the *madhi* is from desire; I do not know what it is."