Narrated 'Aisha:

Allah's Apostle came to me and I told him about the slave-girl (Buraira) Allah's Apostle ﷺ said, "Buy and manumit her, for the Wala is for the one who manumits." In the evening the Prophet got up and glorified Allah as He deserved and then said, "Why do some people impose conditions which are not present in Allah's Book (Laws)? Whoever imposes such a condition as is not in Allah's Laws, then that condition is invalid even if he imposes one hundred conditions, for Allah's conditions are more binding and reliable."

حَدَّثَنَا أَبُو الْيَمَانِ، أَخْبَرَنَا شُعَيْبٌ، عَنِ الزُّهْرِيِّ، قَالَ عُرْوَةُ بْنُ الزُّبَيْرِ قَالَتْ عَائِشَةُ ـ رضى الله عنها دَخَلَ عَلَىَّ رَسُولُ اللَّهِ ﷺ فَذَكَرْتُ لَهُ، فَقَالَ رَسُولُ اللَّهِ ﷺ " اشْتَرِي وَأَعْتِقِي، فَإِنَّ الْوَلاَءَ لِمَنْ أَعْتَقَ ". ثُمَّ قَامَ النَّبِيُّ ﷺ مِنَ الْعَشِيِّ، فَأَثْنَى عَلَى اللَّهِ بِمَا هُوَ أَهْلُهُ، ثُمَّ قَالَ " مَا بَالُ أُنَاسٍ يَشْتَرِطُونَ شُرُوطًا لَيْسَ فِي كِتَابِ اللَّهِ، مَنِ اشْتَرَطَ شَرْطًا لَيْسَ فِي كِتَابِ اللَّهِ فَهْوَ بَاطِلٌ، وَإِنِ اشْتَرَطَ مِائَةَ شَرْطٍ، شَرْطُ اللَّهِ أَحَقُّ وَأَوْثَقُ ".