All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.
If you are able to perform the prayer with a clean diaper, then it is an obligation on you to do so because purifying oneself from Najaasah (impurity) is an obligation for the validity of the prayer. A person who is suffering from urinary incontinence, even though he is permitted to pray while the impurity (urine) is being released, this is permitted for him due to the necessity, and the necessity is measured according to its proper estimate.
If a person is able to remove the diaper that has become impure or to clean it without any great difficulty, then there is no necessity for him that allows him to pray with that impurity.
A Fatwa from the Permanent Committee concerning a question about urinary incontinence reads: “You are obliged to protect yourself from urine so that the impurity does not stain your underwear, and you are obliged to change this diaper if it has become impure.”
Shaykh Ibn Baaz was asked: “I am an old man suffering from urinary incontinence and I wear a diaper. If I am outside my home, I am with people and I can’t change it if it is full of urine. I have performed Wudhoo' (ablution) for Maghrib prayer and prayed without changing the diaper, so what is the ruling on this?“
He answered: “You are obliged to repeat the mentioned prayer as you did not cleanse yourself from urine after the time of the prayer had begun, and you did not change your diaper.”
Allaah Knows best.