Is one's fast considered valid if one ate suhoor, thinking that dawn had not yet broken, but then it became clear that one had eaten for a period of time after the adhan of Fajr?

The majority of scholars hold that if someone eats or drinks, believing night still to prevail, and then it becomes clear that dawn has broken, their fast is invalid, and they must make up the day if it was an obligatory fast, citing the verse: ﴿Then complete the fast until the night.﴾

However, Sheikh al-Islam Ibn Taymiyyah was of the opinion that the fast is valid in this case, considering this view to be sounder and more akin to the principles of Sharia and the indications of the Qur’an and Sunnah. He held that Allah has lifted accountability from the forgetful and the mistaken, and this person is mistaken but not negligent, for he did what Allah permitted and even recommended delaying. Thus, he is more deserving of excuse than the forgetful.

As for voluntary fasting, the madhhab of Imam Ahmad and Al-Shafi’i is that it is not obligatory to complete or make up the fast for someone who broke it, citing the actions of Ibn Umar and Ibn Abbas, may Allah be pleased with them. If you wish to complete the six days of Shawwal fasting, you should fast one day in place of the day that was broken.