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It was narrated that ‘Ali (رضي الله عنه) said: The Messenger of Allah (ﷺ) said: `The urine of a boy is to be sprinkled with water and the urine of a girl is to be washed.` Qatadah said: That is if they are not yet eating solid food; if they are eating solid food then their urine is to be washed in both cases.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَبْدُ الصَّمَدِ بْنُ عَبْدِ الْوَارِثِ، حَدَّثَنَا هِشَامٌ، عَنْ قَتَادَةَ، عَنْ أَبِي حَرْبِ بْنِ أَبِي الْأَسْوَدِ، عَنْ أَبِيهِ، عَنْ عَلِيٍّ، رَضِيَ اللَّهُ عَنْهُ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بَوْلُ الْغُلَامِ يُنْضَحُ عَلَيْهِ وَبَوْلُ الْجَارِيَةِ يُغْسَلُ قَالَ قَتَادَةُ هَذَا مَا لَمْ يَطْعَمَا فَإِذَا طَعِمَا غُسِلَ بَوْلُهُمَا.
sunnah.com