The Book of Miscellany
كتاب المقدمات
(40) Chapter: Kind Treatment towards Parents and establishment of the ties of Blood Relationship
(40) - باب بر الوالدين وصلة الأرحام
Riyad as-Salihin 313 
Abu Hurairah (May Allah be pleased with him) reported:
Messenger of Allah (ﷺ) said, "No son can repay (the kindness shown by his father) unless he finds him a slave and buys him and emancipates him".

[Muslim].

عَنْ أَبِي هُرَيْرَةَ رضي الله عنه، قَالَ: قَالَ رَسُولُ الله صلى الله عليه وسلم ‏ "‏لاَ يَجْزِي وَلَدٌ وَالِدًا إِلاَّ أَنْ يَجِدَهُ مَمْلُوكًا فَيَشْتَرِيَهُ فَيُعْتِقَهُ‏" ‏ ‏(‏‏(‏رواه مسلم‏)‏‏)‏‏.‏
Reference : Riyad as-Salihin 313
In-book reference : Introduction, Hadith 313