Short answer
At Fajr. The fast begins at true dawn (subh sadiq), when Fajr time starts, which is well before sunrise. You must stop eating and drinking by then. The imsak time printed a few minutes earlier in many calendars is a safety margin, not the deadline. All four schools agree.
The fast runs from the start of true dawn until sunset. Eating after true dawn breaks the fast with qada, even if by mistake. Eating while unsure whether dawn has come is makruh.
The four schools agree on this.
Explanation
Allah says: eat and drink until the white thread of dawn becomes distinct from the black thread of night, then complete the fast until the night. When Adi ibn Hatim took this literally and put a white and a black thread under his pillow, the Prophet ﷺ explained that it means the whiteness of day appearing against the blackness of night: the first light of dawn spread along the horizon.
Three times people confuse:
- Imsak: a caution line, often 10 minutes before Fajr in printed timetables. Eating until Fajr is still allowed.
- Fajr (true dawn): the actual start of the fast and of Fajr prayer time. Stop eating here.
- Sunrise: the end of Fajr prayer time. It has nothing to do with the start of the fast.
The adhan. In Madinah, Bilal called an adhan before dawn to wake people, and the Prophet ﷺ told them to keep eating until Ibn Umm Maktum called the adhan at dawn. Today, if your masjid calls Fajr at the start of its time, stop eating when it begins. If it calls later, follow the timetable, not the adhan.
If you ate after Fajr by mistake, the fast is broken: stop at once, hold back for the rest of the day, and make up one day, with no kaffarah. See missing suhoor. Different timetables use slightly different dawn angles, so follow a reliable local one. See suhoor in Ramadan and when the fast ends.
The evidence
Quran, Surah Al-Baqara 2:187
أُحِلَّ لَكُمْ لَيْلَةَ ٱلصِّيَامِ ٱلرَّفَثُ إِلَىٰ نِسَآئِكُمْ ۚ هُنَّ لِبَاسٌۭ لَّكُمْ وَأَنتُمْ لِبَاسٌۭ لَّهُنَّ ۗ عَلِمَ ٱللَّهُ أَنَّكُمْ كُنتُمْ تَخْتَانُونَ أَنفُسَكُمْ فَتَابَ عَلَيْكُمْ وَعَفَا عَنكُمْ ۖ فَٱلْـَٰٔنَ بَٰشِرُوهُنَّ وَٱبْتَغُوا۟ مَا كَتَبَ ٱللَّهُ لَكُمْ ۚ وَكُلُوا۟ وَٱشْرَبُوا۟ حَتَّىٰ يَتَبَيَّنَ لَكُمُ ٱلْخَيْطُ ٱلْأَبْيَضُ مِنَ ٱلْخَيْطِ ٱلْأَسْوَدِ مِنَ ٱلْفَجْرِ ۖ ثُمَّ أَتِمُّوا۟ ٱلصِّيَامَ إِلَى ٱلَّيْلِ ۚ وَلَا تُبَٰشِرُوهُنَّ وَأَنتُمْ عَٰكِفُونَ فِى ٱلْمَسَٰجِدِ ۗ تِلْكَ حُدُودُ ٱللَّهِ فَلَا تَقْرَبُوهَا ۗ كَذَٰلِكَ يُبَيِّنُ ٱللَّهُ ءَايَٰتِهِۦ لِلنَّاسِ لَعَلَّهُمْ يَتَّقُونَ ﴿١٨٧﴾
It has been made permissible for you the night preceding fasting to go to your wives [for sexual relations]. They are clothing for you and you are clothing for them. Allah knows that you used to deceive yourselves, so He accepted your repentance and forgave you. So now, have relations with them and seek that which Allah has decreed for you. And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the sunset. And do not have relations with them as long as you are staying for worship in the mosques. These are the limits [set by] Allah, so do not approach them. Thus does Allah make clear His ordinances to the people that they may become righteous.
The fast begins with the first light of dawn.
Sahih al-Bukhari 1916
Sahih... إنما ذلك سواد الليل وبياض النهار ...
"That only means the blackness of the night and the whiteness of the day."
The white thread is the light of true dawn, not sunrise.
Sahih al-Bukhari 617
Sahih... إن بلالا يؤذن بليل فكلوا واشربوا حتى ينادي ابن أم مكتوم ...
"Bilal calls the adhan while it is still night, so eat and drink until Ibn Umm Maktum calls the adhan."
Eating is allowed until the dawn adhan at the true start of Fajr.
Where the ruling is recorded
- al-Hidaya, Kitab al-Sawm, opening section. The fast is abstaining from dawn to sunset.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Sawm, chapter on what breaks the fast. Eating in doubt about dawn is disliked, and eating after dawn requires qada.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.