وَعَنْ حَفْصَةَ أُمِّ اَلْمُؤْمِنِينَ رَضِيَ اَللَّهُ عَنْهَا
عَنِ اَلنَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «مَنْ لَمْ يُبَيِّتِ اَلصِّيَامَ قَبْلَ اَلْفَجْرِ فَلَا صِيَامَ لَهُ»
“Whoever does not form his intention to fast before to fajr, his fasting will not be accepted.” Related by the five Imams, but At-Tirmidhi and An-Nasai consider it to be related by Hafsah and not connected to the Prophet (sallallahu alayhi wa sallam). Imam Ad-Daraqutni transmitted, “No fasting is accepted for one who does not form the intention (to fast) the night before.”
Bulugh al-Maram 764, Book 5, Fasting, Hadith 7. 1 min read