Hadith: That she brought her small son, who had not yet eaten solid food, to the Messenger of Allah (ﷺ). The Messenger of Allah (ﷺ) seated him on his lap, and he (the child) urinated on his garment. So the Prophet (ﷺ) called for some water, sprinkled it over the affected area, and did not wash it

عَنْ أُمِّ قَيْسٍ بِنْتِ مِحْصَنٍ رَضيَ اللهُ عنها:
أَنَّهَا أَتَتْ بِابْنٍ لَهَا صَغِيرٍ لَمْ يَأْكُلِ الطَّعَامَ إِلَى رَسُولِ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ، فَأَجْلَسَهُ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ فِي حَجْرِهِ، فَبَالَ عَلَى ثَوْبِهِ، فَدَعَا بِمَاءٍ، فَنَضَحَهُ وَلَمْ يَغْسِلْهُ.
[صحيح] - [متفق عليه] - [صحيح البخاري: 223]

’Umm Qays bint Mihsan (may Allah be pleased with her) reported:
That she brought her small son, who had not yet eaten solid food, to the Messenger of Allah (ﷺ). The Messenger of Allah (ﷺ) seated him on his lap, and he (the child) urinated on his garment. So the Prophet (ﷺ) called for some water, sprinkled it over the affected area, and did not wash it.
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Explanation

’Umm Qays bint Mihsan (may Allah be pleased with her) brought her son, who had not yet eaten solid food for being too young, to the Prophet (ﷺ). He (ﷺ) seated the boy on his lap, and the boy urinated on the Prophet's garment. Thereupon, the Prophet (ﷺ) asked for some water, sprinkled it over his garment, and did not wash it.

Benefits from the Hadith

1. The noble character and great humility of the Prophet (ﷺ).
2. Encouragement to show good companionship, humility, and gentleness with children, and to comfort the hearts of adults by honoring their children, seating them on one's lap, and similar acts of kindness.
3. The urine of a male infant is impure, even if he has not yet eaten solid food out of desire for it.
4. The action of the Prophet (ﷺ) is called Nad'h (sprinkling), and it applies specifically to a male infant who has not yet begun eating solid food. As for a female infant, her urine must be washed, even if she is a small infant.
5. The excrement of a male infant who is nourished by his mother's milk must be washed, just like other impurities.
6. Washing requires something additional beyond merely letting the water reach the area.
7. It is preferable to hasten in purifying the place of impurity, both to quickly cleanse oneself from filth and to avoid forgetting it.