I was informed that he (the Holy Prophet) cursed the one who did that (and it was recorded) in his Sahifa (in a document).  

أَنَّهُ سَمِعَ جَابِرَ بْنَ عَبْدِ اللَّهِ يَقُولُ كَتَبَ النَّبِيُّ ﷺ عَلَى كُلِّ بَطْنٍ عُقُولَهُ ثُمَّ كَتَبَ أَنَّهُ لاَ يَحِلُّ لِمُسْلِمٍ أَنْ يَتَوَالَى مَوْلَى رَجُلٍ مُسْلِمٍ بِغَيْرِ إِذْنِهِ ثُمَّ أُخْبِرْتُ أَنَّهُ لَعَنَ فِي صَحِيفَتِهِ مَنْ فَعَلَ ذَلِكَ  

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).  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ مَنْ تَوَلَّى قَوْمًا بِغَيْرِ إِذْنِ مَوَالِيهِ فَعَلَيْهِ لَعْنَةُ اللَّهِ وَالْمَلاَئِكَةِ لاَ يُقْبَلُ مِنْهُ عَدْلٌ وَلاَ صَرْفٌ  

He who took the freed slave as his ally without the consent of his previous master, there is upon him the curse of Allah and that of His angels and that of the whole mankind, and there will not be accepted from him his obligatory acts or supercrogatory acts on the Day of Resurrection. This hadith is narrated through the same chain of transmitters, but with a slight change of words.  

عَنِ النَّبِيِّ ﷺ قَالَ مَنْ تَوَلَّى قَوْمًا بِغَيْرِ إِذْنِ مَوَالِيهِ فَعَلَيْهِ لَعْنَةُ اللَّهِ وَالْمَلاَئِكَةِ وَالنَّاسِ أَجْمَعِينَ لاَ يُقْبَلُ مِنْهُ يَوْمَ الْقِيَامَةِ عَدْلٌ وَلاَ صَرْفٌ وَحَدَّثَنِيهِ إِبْرَاهِيمُ بْنُ دِينَارٍ حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُوسَى حَدَّثَنَا شَيْبَانُ عَنِ الأَعْمَشِ بِهَذَا الإِسْنَادِ غَيْرَ أَنَّهُ قَالَ وَمَنْ وَالَى غَيْرَ مَوَالِيهِ بِغَيْرِ إِذْنِهِمْ  

'Ali b. Abu Talib (Allah be pleased with him) addressed us and said: He who thinks that we (the members of the Prophet's family) read anything else besides the Book of Allah and this Sahifa (and he said that Sahifa was tied to the scabbard of the sword) tells a lie. (This Sahifa) contains (problems) pertaining to the ages of the camels and (the recompense) of the injuries, and it also records the words of the Prophet ﷺ: Medina is a sacred territory from 'Ayr to Thaur (it is most probably Uhud). He who innovates (an act or practice) or gives protection to an innovator, there is a curse of Allah and that of His angels and that of the whole humanity upon him. Allah will not accdpt from him (as a recompense) any obligatory act or supererogatory act, and the responsibility of the Muslims is a joint responsibility; even the lowest in rank can undertake the responsibility (on behalf of others), and he who claims anyone else as his father besides his own father or makes one his ally other than the one (who freed him), there is a curse of Allah. that of His angels and that of the wholemankind upon him. Allah will not accept the obligatory act of the supererogatery act (as a recompense) from him.  

أَنَّ عِنْدَنَا شَيْئًا نَقْرَأُهُ إِلاَّ كِتَابَ اللَّهِ وَهَذِهِ الصَّحِيفَةَ قَالَ وَصَحِيفَةٌ مُعَلَّقَةٌ فِي قِرَابِ سَيْفِهِ فَقَدْ كَذَبَ فِيهَا أَسْنَانُ الإِبِلِ وَأَشْيَاءُ مِنَ الْجِرَاحَاتِ وَفِيهَا قَالَ النَّبِيُّ ﷺ الْمَدِينَةُ حَرَمٌ مَا بَيْنَ عَيْرٍ إِلَى ثَوْرٍ فَمَنْ أَحْدَثَ فِيهَا حَدَثًا أَوْ آوَى مُحْدِثًا فَعَلَيْهِ لَعْنَةُ اللَّهِ وَالْمَلاَئِكَةِ وَالنَّاسِ أَجْمَعِينَ لاَ يَقْبَلُ اللَّهُ مِنْهُ يَوْمَ الْقِيَامَةِ صَرْفًا وَلاَ عَدْلاً وَذِمَّةُ الْمُسْلِمِينَ وَاحِدَةٌ يَسْعَى بِهَا أَدْنَاهُمْ وَمَنِ ادَّعَى إِلَى غَيْرِ أَبِيهِ أَوِ انْتَمَى إِلَى غَيْرِ مَوَالِيهِ فَعَلَيْهِ لَعْنَةُ اللَّهِ وَالْمَلاَئِكَةِ وَالنَّاسِ أَجْمَعِينَ لاَ يَقْبَلُ اللَّهُ مِنْهُ يَوْمَ الْقِيَامَةِ صَرْفًا وَلاَ عَدْلاً