As for the urine of the boy, it is sufficient to sprinkle it with water if the child does not eat food, based upon the narration of Umm Qais bint Mihsan, may Allaah be pleased with her. She brought her small son, who had not yet begun eating, to the Messenger of Allaah (Sallallaahu alayhi wasallam) and he sat him in a room, and he urinated on his garment, so he called for water and he sprinkled it, without washing it. [Al-Bukhari no. 233 and Muslim no. 278]
The meaning of Nadh (the word used in the Hadith) is to wet with water, even if it does not run off to the ground, and it does not require soaking. It is understood from this that if the baby has eaten food, the urine must be washed out. As for the urine of the girl, it must be washed out, based upon the narration of Lubabah bint Al-Harith, who said:
“Al-Husain bin ‘Ali was in the room of the Prophet (Sallallaahu alayhi wasallam) and he urinated upon him (Sallallaahu alayhi wasallam), so I said: ‘Wear another garment, and give me your Izar while I wash it.’ He said:
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“Only the urine of the girls must be washed, and the boy’s urine may be sprinkled.” [Abu Dawud no. 375]
This is the guidance of the Messenger (Sallallaahu alayhi wasallam) regarding the ruling on the urine of the boy and the girl. From what the questioner mentioned about drying his garment in the sun on which the child has urinated and then praying in it when it is dry, it is clear that the sun does not purify it and that prayer in it is not correct, as we have said.