The mother of the believers, Hafsah (RAA) narrated that The Messenger of Allah (ﷺ) said:

“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-Nasa’i consider it to be related by Hafsah and not connected to the Prophet (ﷺ). Imam Ad-Daraqutni transmitted, "No fasting is accepted for one who does not form the intention (to fast) the night before.”

وَعَنْ حَفْصَةَ أُمِّ اَلْمُؤْمِنِينَ رَضِيَ اَللَّهُ عَنْهَا, عَنِ اَلنَّبِيِّ ‏- صلى الله عليه وسلم ‏-قَالَ: { مَنْ لَمْ يُبَيِّتِ اَلصِّيَامَ قَبْلَ اَلْفَجْرِ فَلَا صِيَامَ لَهُ } رَوَاهُ اَلْخَمْسَةُ, وَمَالَ النَّسَائِيُّ وَاَلتِّرْمِذِيُّ إِلَى تَرْجِيحِ وَقْفِهِ, وَصَحَّحَهُ مَرْفُوعًا اِبْنُ خُزَيْمَةَ وَابْنُ حِبَّانَ [1]‏ .‏ وَلِلدَّارَقُطْنِيِّ: { لَا صِيَامَ لِمَنْ لَمْ يَفْرِضْهُ مِنَ اَللَّيْلِ } [2]‏ .‏