Hadith 634
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ كَانَ يَقُولُ لاَ يَصُومُ إِلاَّ مَنْ أَجْمَعَ الصِّيَامَ قَبْلَ الْفَجْرِ .
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
Mauquf Sahih (Salim al-Hilali)
Muwatta Malik 634 · Book 18, Hadith 5