It was narrated from 'Ali that :
The Prophet said concerning the urine of a nursing infant: "Water should be sprinkled over the urine of a boy, and the urine of a girl should be washed." Abul-Hasan bin Salamah said: "Ahmad bin Musa bin Ma'qil narrated to us that Abul-Yaman Al-Misri said: 'I asked Shafi'i about the Hadith of the Prophet, "Water should be sprinkled over the urine of a baby boy, and the urine of a baby girl should be washed," when the two types of water (urine) are the same. He said, "This is because the urine of the boy is of water and clay, but the urine of the girl is of flesh and blood." Then he said to me: "Did you understand?" I said: "No." He said: "When Allah the Most High created Adam, He created Eve (Hawwa') from his short rib, so the boy's urine is from water and clay, and the girl's urine is from flesh and blood." Then he said to me: "Did you understand?" I said: "Yes." He said: "May Allah cause you benefit from this."
حَدَّثَنَا حَوْثَرَةُ بْنُ مُحَمَّدٍ، وَمُحَمَّدُ بْنُ سَعِيدِ بْنِ يَزِيدَ بْنِ إِبْرَاهِيمَ، قَالاَ حَدَّثَنَا مُعَاذُ بْنُ هِشَامٍ، أَنْبَأَنَا أَبِي، عَنْ قَتَادَةَ، عَنْ أَبِي حَرْبِ بْنِ أَبِي الأَسْوَدِ الدِّيلِيِّ، عَنْ أَبِيهِ، عَنْ عَلِيٍّ، أَنَّ النَّبِيَّ ـ صلى الله عليه وسلم ـ قَالَ فِي بَوْلِ الرَّضِيعِ ‏"‏ يُنْضَحُ بَوْلُ الْغُلاَمِ وَيُغْسَلُ بَوْلُ الْجَارِيَةِ ‏"‏ ‏.‏

قَالَ أَبُو الْحَسَنِ بْنُ سَلَمَةَ حَدَّثَنَا أَحْمَدُ بْنُ مُوسَى بْنِ مَعْقِلٍ، حَدَّثَنَا أَبُو الْيَمَانِ الْمِصْرِيُّ، قَالَ سَأَلْتُ الشَّافِعِيَّ عَنْ حَدِيثِ النَّبِيِّ، ـ صلى الله عليه وسلم ـ ‏"‏ يُرَشُّ مِنْ بَوْلِ الْغُلاَمِ وَيُغْسَلُ مِنْ بَوْلِ الْجَارِيَةِ ‏"‏ ‏.‏ وَالْمَاءَانِ جَمِيعًا وَاحِدٌ قَالَ لأَنَّ بَوْلَ الْغُلاَمِ مِنَ الْمَاءِ وَالطِّينِ وَبَوْلَ الْجَارِيَةِ مِنَ اللَّحْمِ وَالدَّمِ ‏.‏ ثُمَّ قَالَ لِي فَهِمْتَ أَوْ قَالَ لَقِنْتَ قَالَ قُلْتُ لاَ ‏.‏ قَالَ إِنَّ اللَّهَ تَعَالَى لَمَّا خَلَقَ آدَمَ خُلِقَتْ حَوَّاءُ مِنْ ضِلَعِهِ الْقَصِيرِ فَصَارَ بَوْلُ الْغُلاَمِ مِنَ الْمَاءِ وَالطِّينِ وَصَارَ بَوْلُ الْجَارِيَةِ مِنَ اللَّحْمِ وَالدَّمِ ‏.‏ قَالَ قَالَ لِي فَهِمْتَ قُلْتُ نَعَمْ ‏.‏ قَالَ لِي نَفَعَكَ اللَّهُ بِهِ ‏.‏
GradeHasan (Darussalam)
English reference : Vol. 1, Book 1, Hadith 525
Arabic reference : Book 1, Hadith 567