Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that on the day of Hunayn or Khaybar he (peace and blessings be upon him) took hold of some camel hair and said: "O people! Not even this much of what Allah has returned to you is lawful for me, except the one-fifth, and the one-fifth too is returned to you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12932