the Prophet said: "Whoever did not decide to fast before Fajr then there is no fast for him."

عَنِ النَّبِيِّ ﷺ قَالَ مَنْ لَمْ يُجْمِعِ الصِّيَامَ قَبْلَ الْفَجْرِ فَلاَ صِيَامَ لَهُ