‘Amir Al Sha’bi said “The Prophet (ﷺ) had a special portion in the booty called safi. This would be a slave if he desired or a slave girl if he desired or a horse if he desired. He would choose it before taking out the fifth.”
حَدَّثَنَا مُحَمَّدُ بْنُ كَثِيرٍ، أَخْبَرَنَا سُفْيَانُ، عَنْ مُطَرِّفٍ، عَنْ عَامِرٍ الشَّعْبِيِّ، قَالَ كَانَ لِلنَّبِيِّ صلى الله عليه وسلم سَهْمٌ يُدْعَى الصَّفِيَّ إِنْ شَاءَ عَبْدًا وَإِنْ شَاءَ أَمَةً وَإِنْ شَاءَ فَرَسًا يَخْتَارُهُ قَبْلَ الْخُمُسِ .
AbuDaud
Tribute, Spoils, and Rulership (Kitab Al-Kharaj, Wal-Fai' Wal-Imarah) • Hadith 2991