وَكُلُوا۟ وَٱشْرَبُوا۟ حَتَّىٰ يَتَبَيَّنَ لَكُمُ ٱلْخَيْطُ ٱلْأَبْيَضُ مِنَ ٱلْخَيْطِ ٱلْأَسْوَدِ مِنَ ٱلْفَجْر
And eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your Saum (fast) till the nightfall.
Shaykh ‘Ubayd al-Jaabiri حفظه الله said:
It is allowed to have food and drink and to partake in that which is permissible during the night for the fasting person once he completes that day, once the sun has set then it is allowed for him to eat and drink and partake in that which is permissible for him, right up until the fajr.
Hadith of the Adi bin Hatim رضي الله عنه:
When the above verses were revealed: ‘Until the white thread appears to you, distinct from the black thread,’ I took two (hair) strings, one black and the other white, and kept them under my pillow and went on looking at them throughout the night but could not make anything out of it. So, the next morning I went to Allah’s Messenger (ﷺ) and told him the whole story. He explained to me, “That verse means the darkness of the night and the whiteness of the dawn.”
(Bukhaari and Muslim)
Narrated Sahl bin Saad:
When the following verses were revealed: ‘Eat and drink until the white thread appears to you, distinct from the black thread’ and of dawn was not revealed, some people who intended to fast tied black and white threads to their legs and went on eating until they differentiated between the two. Allah then revealed the words, ‘of dawn’, and it became clear that meant night and day.
(Bukhaari and Muslim)
The hadith of Adee ibn Hateem and Sahl ibn Saad clearly show the ruling with regards a person that is excused due to ignorance and that is with the condition that thing they are ignorant of something that which other than themselves can also be hidden from them.
To clarify that further, the story itself;
The Shaykh mentioned in the story of Adee ibn Hateem, when this verse was revealed eat and drink until you are able to distinguish between the black thread and the white thread of night.
The word from Fajr had not been revealed so he misunderstood the verse and he took the white thread and the black thread and put it underneath his pillow and was trying to distinguish between them to find out when does the fajr comes in, and this was his misunderstanding and the Prophet ﷺ clarified to him the next day, What is meant by that verse is not white thread (actual white thread) and the black thread, but rather it is the white and black thread of the night, meaning of the horizon and so he will be excused for his ignorance because other than him also was ignorant of this verse because the rest of the verse “minal fajr” has not been revealed, so they misunderstood the verse and Messenger ﷺ did not ordered him to repeat his fast.