Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.4181
Fataawa Ramadhaan – Volume 2, Page 569, Fatwa No.553
Question: I awoke one night, in the blessed month of Ramadhaan, and drank some water that was beside my bed and then returned back to sleep. When I awoke, I found that the people were returning from the Fajr prayer and the morning was bright and clear. So, in this state I began asking myself when it was that I had drank the water. Was it before, during or after [Fajr]?
I was half-asleep but, as for the actual act of drinking, then I am sure about that. So, do I make this fast up or not, O Shaykh?
Response: If the situation is as described, then the correct position is that you must assume it was night and therefore your fast is correct, until such time that it becomes evident to you that the act of drinking had occurred after Fajr had entered. Then, you have to make up the fast for that day.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.