Is it obligatory to repeat the Fajr prayer if I prayed it while unsure about the the sun's rising, and should I start with the obligatory prayer?

Prayer is a great pillar of Islam, and it is the first thing a Muslim will be held accountable for. Whoever neglects it has failed and lost, and severe warnings have been established against those who are negligent in performing it, as Allah Almighty says: "Then succeeded them a generation who neglected prayers and pursued desires; so they are going to meet evil," and His saying: "So woe to those who pray, [but] who are heedless of their prayer." Part of maintaining it is performing it at its prescribed time. There is no need to repeat a prayer that has already been performed. If you wake up before sunrise with enough time for four rak'ahs or more, start with the Fajr Sunnah (definition) then the obligatory prayer. If there is only enough time for two rak'ahs, limit yourself to the obligatory prayer, and you can make up the Sunnah after the sun has risen. If you wake up after sunrise, start with the Fajr Sunnah before the obligatory prayer. You should take specific measures to wake up, such as using an alarm clock or asking someone to wake you.