Sunan an-Nasa'i, Volume 3, Book 22, Hadith 2341 (DarusSalam) ⓘ
It was narrated that Hafsah said: "There is no fast for the one who does not intend to fast before dawn.
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ حَمْزَةَ بْنِ عَبْدِ اللَّهِ بْنِ عُمَرَ، عَنْ حَفْصَةَ، قَالَتْ لاَ صِيَامَ لِمَنْ لَمْ يُجْمِعِ الصِّيَامَ قَبْلَ الْفَجْرِ .
also referenced under: Hadith 2339 (Reference) & Book 22, Hadith 250 (In-Book)