كتاب العتق
20  
The Book of Emancipating Slaves
(5)
Chapter: The prohibition of a manumitted slave taking anyone as a Mawla except the one who manumitted him
(5)
باب تَحْرِيمِ تَوَلِّي الْعَتِيقِ غَيْرَ مَوَالِيهِ ‏‏
Sahih Muslim 1508 a

Abu Huraira (Allah be pleased with him) reported that Allah's Messenger (ﷺ) said:

He who takes anyone as his ally without the consent of his previous master, there will be the curse of Allah and that of His angels upon him, and neither, any obligatory act of his nor the supererogatory one will be accepted (by Allah).
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا يَعْقُوبُ، - يَعْنِي ابْنَ عَبْدِ الرَّحْمَنِ الْقَارِيَّ - عَنْ سُهَيْلٍ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ مَنْ تَوَلَّى قَوْمًا بِغَيْرِ إِذْنِ مَوَالِيهِ فَعَلَيْهِ لَعْنَةُ اللَّهِ وَالْمَلاَئِكَةِ لاَ يُقْبَلُ مِنْهُ عَدْلٌ وَلاَ صَرْفٌ ‏"‏ ‏.‏
Reference : Sahih Muslim 1508a
In-book reference : Book 20, Hadith 22
USC-MSA web (English) reference : Book 9, Hadith 3599
  (deprecated numbering scheme)