Amr bin Shu'ayb narrates from his father, and he from his grandfather (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said: "Nothing of this fay' is for me except the khumus (one-fifth), and the khumus is returned among you."
Imam al-Shafi'i (may Allah have mercy on him) said: By "the khumus" he means his right from the khumus, and by his words "returned among you" he means: (spent) in your interests.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الصدقات / 13179