I had to make up for a day of fasting that I missed last Ramadan and I actually did that before the advent of the current Ramadan. Here is my question: Having performed the Fajr Prayer, I slept and when I woke up, I looked at the clock and thought that the sun had already set. So, I drank a cup of water and then returned to sleep again. When I woke up again, I did not remember whether it had in fact been before or after sunset when I drank the water. I did not remember the exact time because I was sleepy when I looked at the clock. Should I make up for this day after ending this current Ramadan? Thank you.
All perfect praise be to Allah, the Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His Slave and Messenger.
If you drank water while being unsure of whether or not it was still daytime (i.e., the two possibilities, that the sun set or not, were of the same degree), then undoubtedly you would not be free from liability and you would be required to make up for that day because the basic premise is that it was still daytime and this premise would not have changed unless you were certain or most likely believed that it had ended.
If you drank water while thinking that the sun set and the day ended, you would not be required to make up for that day because the opposite was not proved to you.
Finally, we would like to state the following: It is impermissible for a Muslim to delay the obligatory prayer beyond its prescribed time without a valid excuse. Rather, every accountable (sane and pubescent) Muslim (man and woman) is obliged to perform the prayers on time. Missing prayers due to oversleeping is a valid excuse only if one exerted all means to wake up and perform them on time and nonetheless slept through them. However, one is sinful if he/she misses a prayer due to failing to utilize all available means to wake up and perform the prayer on time.
And Allah Knows best.