Muwatta Malik, Book 18, Hadith 637 (In-Book) ⓘ
Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say, "Only some one who makes the intention to fast before dawn (actually) fasts."
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ كَانَ يَقُولُ لاَ يَصُومُ إِلاَّ مَنْ أَجْمَعَ الصِّيَامَ قَبْلَ الْفَجْرِ .
also referenced under: Book 18, Hadith 5 (USC-MSA)