Sunan an-Nasa'i, Book 22, Hadith 247 (In-Book) ⓘ
Hamzah bin 'Abdullah bin 'Umar narrated that his father said: "Hafsah, the wife of the Prophet said: 'There is no fast for the one who does not intend (to fast) before dawn."'
أَخْبَرَنَا الرَّبِيعُ بْنُ سُلَيْمَانَ، قَالَ حَدَّثَنَا ابْنُ وَهْبٍ، قَالَ أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، قَالَ أَخْبَرَنِي حَمْزَةُ بْنُ عَبْدِ اللَّهِ بْنِ عُمَرَ، عَنْ أَبِيهِ، قَالَ قَالَتْ حَفْصَةُ زَوْجُ النَّبِيِّ صلى الله عليه وسلم لاَ صِيَامَ لِمَنْ لَمْ يُجْمِعْ قَبْلَ الْفَجْرِ .
also referenced under: Hadith 2336 (Reference) & Volume 3, Book 22, Hadith 2338 (DarusSalam)