Shaykh Salih al-Fawzan explains that if a person prays without wudu and later remembers, he must make ablution and repeat the prayers, since purification is a condition for valid prayer and is not excused by forgetfulness.
I was travelling and the time for maghrib entered. So we prayed maghrib and ʿisha. Then after two hours, I remembered that I did not have ablution. So what is the ruling on that?
The ruling is that you perform ablution and repeat the two prayers again because they are not accepted from you since you were not pure. And purification is a condition for the validity of prayer. And [the condition] ablution does not waive from you due to forgetfulness.
