The urine of a girl should be washed, but the urine of a boy should be splashed with water

Abu As-Samh (may Allah be pleased with him) mentioned that he used to serve the Prophet (may Allah's peace and blessings be upon him) and that whenever he wanted to take a bath, he would ask Abu As-Samh to turn his back. So he would turn his back towards the Prophet (may Allah's peace and blessings be upon him) screening him with his body so that neither him nor others could see the Messenger of Allah (may Allah’s peace and blessings be upon him).

Then he mentioned an incident that took place while he was with the Prophet (may Allah’s peace and blessings be upon him). Abu As-Samh brought Al-Hasan or Al-Husayn to the Prophet (may Allah's peace and blessings be upon him) and the boy (who was still infant) urinated on his chest. When Abu As-Samh wanted to wash it, the Messenger of Allah (may Allah’s peace and blessings be upon him) explained to him that splashing water over the urine of the baby boy (male infant) who is not yet eating food is sufficient to purify it, but the urine of a girl must be washed even if she is still infant.

The scholars have mentioned some reasons for this difference between the urine of the boy and that of the girl, including the following:

1. Usually, the male baby is carried a lot by both men and women, so washing his urine would be too difficult, so this concession was made.

2. Unlike the urine of the girl, the urine of a boy spatters over here and there, so it is extremely difficult to thoroughly wash it.

3. The urine of the female is more putrid than the urine of the nursing boy. The reason for that is that the temperature of the urine from the male is hotter, with less stench. High temperature makes the smell of urine mild and removes what might happen with humidity.