Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.009A.

There is agreement among the scholars that these objects are impure. But, a slight amount of vomit (commonly understood as a small amount of liquid) and the urine of an unweaned male baby are overlooked and pardoned. It is sufficient just to sprinkle water over the urine of an unweaned male baby. This is based on the hadith of Umm Qais. She came to the Messenger of Allah with her unweaned son. After a while, the baby urinated in the Prophet’s lap. The Prophet, upon whom be peace, called for some water, which he sprinkled over his clothes, and did not give them a complete washing. This is related by al-Bukhari and Muslim.

‘Ali narrated that the Messenger of Allah said, “The urine of a baby boy should have water sprinkled upon it. The urine of a baby girl is to be washed off.” Says Qatadah, “This refers to a male baby that has not yet begun to eat. If he already eats, then the garment is to be washed.”

This hadith is related by Ahmad, Abu Dawud, at-Tirmizhi and Ibn Majah. In al-Fath, Ibn Hajr says its chain is sahih.

Sprinkling is sufficient as long as the boy is still nursing. If he eats solid food, his urine must be washed from the clothes and body. There is no disagreement on this latter point. Perhaps the reason for this exemption to the male baby’s urine is that people have a tendency to carry their male babies around, and it would have been difficult to clean the clothes after their frequent urinations.