It is narrated from Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) that on the day of Hunayn the Prophet (peace and blessings be upon him) took a tuft of hair from the side of a camel and said: "O people! Of what Allah has restored to you, not even the like of this is lawful for me except the khumus (one-fifth), and the khumus too is returned to you."
That is, and Allah knows best: it is returned (to be spent) in your interests.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12747