عَن حَفْصَةَ رَضِيَ اللَّهُ عَنْهَا
قَالَتْ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنْ لَمْ يَجْمَعِ الصِّيَامَ قَبْلَ الْفَجْرِ فَلَا صِيَامَ لَهُ»
Hafsah reported God’s messenger as saying, “He who does not before dawn express his intention of fasting is not credited with it.” Tirmidhi, Abu Dawud, Nasai and Darimi transmitted it. Abu Dawud said that Mamar, az-Zubaidi, Ibn Uyaina and Yunus al-Aili, all of whom got it on Zuhri’s authority, traced it no farther back than Hafsah.
Mishkat al-Masabih 1897, Book 8, Fasting, Hadith 31. 1 min read