Does the rising of the sun nullify ablution, or is it the entry of the Dhuhr prayer time that nullifies the ablution of the Fajr prayer?

If you wake up for prayer, perform it promptly if there is sufficient time. Scholars have differed regarding the invalidation of the ablution of a person with an excuse due to the expiration of the prayer time. The Shafi'is, in one view, hold that it is not invalidated, and this opinion is strong and lenient, and there is no objection to adopting it. The Hanbalis are of the view that the ablution of a person with an excuse for the Fajr prayer is sufficient until the time for Dhuhr prayer begins, and it is not invalidated by the sunrise. So, whenever you wake up, hasten to perform the prayer and do not delay it until its time expires without a valid excuse.