Ibn Mansur said: "I said to Ahmad: 'The Prophet (ﷺ) gave the Nafl when he divided the fourth, after the Khumus, and when he was returning (he gave) the third from the Khumus.' So he said: 'The Khumus is taken, and then the Nafl is given from what remains, nothing beyond this.''"
[Abu 'Eisa said:] This Hadith is (understood) as Ibn Musayyab said: "The Nafl is from te Khumus." Ishaq said as he said.
In-book : Book 21, Hadith 19
English translation : Vol. 3, Book 19, Hadith 1561