The Book of Fasting
22/66. Chapter:
If a person did not form the intention of fasting from the night before, can he observe a voluntary fast that day?
Sunan an-Nasai
أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنَا يَحْيَى، عَنْ يَزِيدَ، قَالَ حَدَّثَنَا سَلَمَةُ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ لِرَجُلٍ " أَذِّنْ - يَوْمَ عَاشُورَاءَ - مَنْ كَانَ أَكَلَ فَلْيُتِمَّ بَقِيَّةَ يَوْمِهِ وَمَنْ لَمْ يَكُنْ أَكَلَ فَلْيَصُمْ " .
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 2321In-book reference : Book 22, Hadith 232English translation : Vol. 3, Book 22, Hadith 2323