Sunan an-Nasa'i, Hadith 2342 ⓘ
It was narrated that Ibn 'Umar said: "If a man does not intend to fast from the night before, he should not fast."
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا الْمُعْتَمِرُ، قَالَ سَمِعْتُ عُبَيْدَ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ إِذَا لَمْ يُجْمِعِ الرَّجُلُ الصَّوْمَ مِنَ اللَّيْلِ فَلاَ يَصُمْ .
also referenced under: Book 22, Hadith 253 (In-Book) & Volume 3, Book 22, Hadith 2344 (DarusSalam)