An-Nu'man ibn Bashir said that his father had carried him to the Messenger of Allah, may Allah bless him and grant him peace. He said, 'Messenger of Allah, I testify to you that I have given an-Nu'man such-and-such. (It was a slave). The Prophet (ﷺ) asked, "Have you given each of your children the same"?" "No," he replied. He said, "Then testify to someone other than me." Then the Prophet (ﷺ) asked, "Do you not want to show equal kindness to all of them?" "Indeed I do," he replied. He said, "Then do not do it."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ سَلاَمٍ، قَالَ: أَخْبَرَنَا عَبْدُ الأَعْلَى بْنُ عَبْدِ الأَعْلَى الْقُرَشِيُّ، عَنْ دَاوُدَ بْنِ أَبِي هِنْدَ، عَنْ عَامِرٍ، أَنَّ النُّعْمَانَ بْنَ بَشِيرٍ حَدَّثَهُ، أَنَّ أَبَاهُ انْطَلَقَ بِهِ إِلَى رَسُولِ اللهِ صلى الله عليه وسلم يَحْمِلُهُ فَقَالَ: يَا رَسُولَ اللهِ، إِنِّي أُشْهِدُكَ أَنِّي قَدْ نَحَلْتُ النُّعْمَانَ كَذَا وَكَذَا، فَقَالَ: أَكُلَّ وَلَدَكَ نَحَلْتَ؟ قَالَ: لاَ، قَالَ: فَأَشْهِدْ غَيْرِي، ثُمَّ قَالَ: أَلَيْسَ يَسُرُّكَ أَنْ يَكُونُوا فِي الْبِرِّ سَوَاءً؟ قَالَ: بَلَى، قَالَ: فَلاَ إِذًا.
sunnah.com