Allah s.w.t. stated in the Quran:

وَكُلُوا۟ وَٱشْرَبُوا۟ حَتَّىٰ يَتَبَيَّنَ لَكُمُ ٱلْخَيْطُ ٱلْأَبْيَضُ مِنَ ٱلْخَيْطِ ٱلْأَسْوَدِ مِنَ ٱلْفَجْرِ ۖ ثُمَّ أَتِمُّوا۟ ٱلصِّيَامَ إِلَى ٱلَّيْلِ

Which means: “And eat and drink until the white thread of the dawn becomes distinct from the black thread; then complete the fast up to the night.” (Surah al-Baqarah, verse 187)

This verse shows that the period of fasting begins when we enter the time for Subuh. During Imsak (10 minutes before Subuh) it is recommended for us to stop eating and drinking, taking into account that Subuh is approaching soon.

However, it is not prohibited for us to eat and drink during Imsak, especially if you woke up late for Sahur. As long as you stop eating and drinking before Subuh, then your fast is valid.