ENOfficial
Hafsa 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, Nasa’i and Darimi transmitted it. Abu Dawud said that Ma'mar, az-Zubaidi, Ibn ‘Uyaina and Yunus al-Aili, all of whom got it on Zuhri’s authority, traced it no farther back than Hafsa.
Source: sunnah.com
Other translations
Arabic
عَن حَفْصَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنْ لَمْ يَجْمَعِ الصِّيَامَ قَبْلَ الْفَجْرِ فَلَا صِيَامَ لَهُ» . رَوَاهُ التِّرْمِذِيُّ وَأَبُو دَاوُدَ وَالنَّسَائِيُّ وَالدَّارِمَيُّ وَقَالَ أَبُو دَاوُد: وَقفه على حَفْصَة معمر والزبيدي وَابْنُ عُيَيْنَةَ وَيُونُسُ الَأَيْلِيُّ كُلُّهُمْ عَنِ الزُّهْرِيِّ
sunnah.com