Musannaf Ibn Abi Shayba 28:778

Sufyan narrated to us, from Qays ibn Muslim, from al-Hasan ibn Muhammad ibn Ali, who said: I asked him about His saying: "And know that whatever thing you take as spoils, then indeed for Allah is one-fifth of it." He said: This is an opening of speech. Allah has no share; to Allah belongs the world and the Hereafter.