Sunan an-Nasa'i, Hadith 2339 ⓘ
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: Book 22, Hadith 250 (In-Book) & Volume 3, Book 22, Hadith 2341 (DarusSalam)