Ghusl First

Question :

(Let us suppose that) I wake up just before sunrise and find that I am in a state of Janabah; if I go and make Ghusl, the sun will rise, so should I make Tayammum, or Ghusl and then pray?

Answer:

You must perform Ghusl and complete your cleansing, then pray. You may not make Tayammum when the situation is as you have described it, because the one who forgets and the sleeper are ordered to pray without delay and fulfill its requirements upon remembering or waking, according to the Hadith of the Prophet in which he said:
"Whoever slept through prayer or forgot it, he must pray it as soon as he remembers, and there is no expiation for it except this.

And it is well known that there is no prayer without purification, as the Prophet said:
Prayer without purification is not accepted.

And whoever has access to water must purify himself with that, but if he did not find any, he may pray with Tayammum, based upon the Words of Allah:

... and you find no water, then perform Tayammum with clean earth and rub therewith your faces and hands.

The Verse is from Surah Al-Ma'idah. You must be very careful with your prayers and be meticulous in their performance by placing an alarm clock at your head, or giving one of your family members the duty of waking you when the time for prayer comes, so that you fulfill the obligation of prayer which Allah has imposed upon you, along with your Muslim brothers in the Houses of Allah, the Almighty, the All-Powerful, and so that you are safe from any comparison with the hypocrites who delay their prayers, and do perform them except in a lazy manner. May Allah protect us and you and all of the Muslims from their ways and their characteristics. And Allah is the Granter of success.

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 2 Pages 61-62