It was narrated that Musa bin Abi 'Aishah said:"I asked Yahya bin Al-Jazzar about this Verse: and know that whatever of spoils of war that you may gain, verily, one-fifth of it is assigned to Allah, and to the Messenger". He said: "I said: 'How much of the Khumus did he Prophet take?' He said: 'One-fifth of the Khumus."'
أَخْبَرَنَا عَمْرُو بْنُ يَحْيَى بْنِ الْحَارِثِ، قَالَ حَدَّثَنَا مَحْبُوبٌ، قَالَ أَنْبَأَنَا أَبُو إِسْحَاقَ، عَنْ مُوسَى بْنِ أَبِي عَائِشَةَ، قَالَ سَأَلْتُ يَحْيَى بْنَ الْجَزَّارِ عَنْ هَذِهِ الآيَةِ، { وَاعْلَمُوا أَنَّمَا غَنِمْتُمْ مِنْ شَىْءٍ فَأَنَّ لِلَّهِ خُمُسَهُ وَلِلرَّسُولِ } قَالَ قُلْتُ كَمْ كَانَ لِلنَّبِيِّ صلى الله عليه وسلم مِنَ الْخُمُسِ قَالَ خُمُسُ الْخُمُسِ .
Nesai
The Book of Distribution of Al-Fay' • Hadith 4144