(66) باب إِذَا لَمْ يُجْمِعْ مِنَ اللَّيْلِ هَلْ يَصُومُ ذَلِكَ الْيَوْمَ مِنَ التَّطَوُّعِ
It was narrated that Yazid said:
"Salamah told us that the Messenger of Allah said to a man: 'Announce the day of Ashura. Whoever has eaten let him not eat for the rest of the day, and whoever has not eaten, let him fast."'
Reference: Sunan an-Nasa'i 2321
In-book reference: Book 22, Hadith 232
English translation: Vol. 3, Book 22, Hadith 2323