Summary: if it is Ramadan, then he abstains from eating the rest of the day and owes a make-up day. If it is a voluntary fast, then he continues fasting and does not owe a day.
Ibn al-Qasim said, “I asked Malik about one who has a doubt about Fajr in Ramadan and does not know if he ate in it or not. Malik said, he owes a day (but he continues to abstain from eating and drinking for the day). Malik hated that a man eat while doubting the time. Al-Qasim ibn Muhammad said, “However, if the fast is voluntary then he continues fasting and does not owe a day.”
This happened in the time of Umar and while eating the mu’adhin came and said the sun rose. Umar said, “Make up a day.”
Source: Al-Mudawanna, Chapter of Fasting, vol 1, pp 266.
قُلْتُ: مَا قَوْلُ مَالِكٍ فِيمَنْ شَكَّ فِي الْفَجْرِ فِي رَمَضَانَ
فَلَمْ يَدْرِ أَكَلَ فِيهِ أَوْ لَمْ يَأْكُلْ؟
قَالَ، قَالَ مَالِكٌ: عَلَيْهِ قَضَاءُ يَوْمٍ مَكَانَهُ.
قُلْتُ: وَكَانَ مَالِكٌ يَكْرَهُ لِلرَّجُلِ أَنْ يَأْكُلَ إذَا شَكَّ فِي الْفَجْرِ؟
قَالَ: نَعَمْ.
قال الْقَاسِمُ بْنُ مُحَمَّدٍ: إنْ كَانَ فِي فَرِيضَةٍ فَلْيَصُمْ ذَلِكَ الْيَوْمَ وَلِيَقْضِهِ يَوْمًا مَكَانَهُ وَإِنْ كَانَ فِي تَطَوُّعٍ فَلْيَصُمْ ذَلِكَ الْيَوْمَ وَلَا يَقْضِيهِ.
قَالَ ابْنُ وَهْبٍ وَحَدَّثَنِي سُفْيَانُ الثَّوْرِيُّ عَنْ زِيَادِ بْنِ عِلَاقَةَ عَنْ بِشْرِ بْنِ قَيْسٍ قَالَ: كُنَّا عِنْدَ عُمَرَ بْنِ الْخَطَّابِ فَأَتَى بِسَوِيقٍ فَأَصَبْنَا مِنْهُ وَحَسَبْنَا أَنَّ الشَّمْسَ قَدْ غَابَتْ، فَقَالَ الْمُؤَذِّنُ: قَدْ طَلَعَتْ الشَّمْسُ، فَقَالَ عُمَرُ: فَاقْضُوا يَوْمًا مَكَانَهُ.