When a slave looks to the welfare of his master and worships Allah well, he has two rewards for him.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِنَّ الْعَبْدَ إِذَا نَصَحَ لِسَيِّدِهِ وَأَحْسَنَ عِبَادَةَ اللَّهِ فَلَهُ أَجْرُهُ مَرَّتَيْنِ  

This hadith has been narrated on the authority of Ibn 'Umar through another chain of transmitters.  

عَنِ النَّبِيِّ ﷺ بِمِثْلِ حَدِيثِ مَالِكٍ  

For a faithful slave there are two rewards. By him in Whose hand is the life of Abu Huraira, but for Jihad in the cause of Allah, and Pilgrimage and kindness to my mother, I would have preferred to die as a slave. He (one of the narrators in the chain of transmitters) said: This news reached us that Abu Huraira did not perform Pilgrimage until his mother died for (keeping himself constantly) in her service.  

قَالَ رَسُولُ اللَّهِ ﷺ لِلْعَبْدِ الْمَمْلُوكِ الْمُصْلِحِ أَجْرَانِ وَالَّذِي نَفْسُ أَبِي هُرَيْرَةَ بِيَدِهِ لَوْلاَ الْجِهَادُ فِي سَبِيلِ اللَّهِ وَالْحَجُّ وَبِرُّ أُمِّي لأَحْبَبْتُ أَنْ أَمُوتَ وَأَنَا مَمْلُوكٌ قَالَ وَبَلَغَنَا أَنَّ أَبَا هُرَيْرَةَ لَمْ يَكُنْ يَحُجُّ حَتَّى مَاتَتْ أُمُّهُ لِصُحْبَتِهَا قَالَ أَبُو الطَّاهِرِ فِي حَدِيثِهِ لِلْعَبْدِ الْمُصْلِحِ وَلَمْ يَذْكُرِ الْمَمْلُوكَ  

This hadith has been transmitted on the authority of Abu Tahir but with a slight variation of words.  

عَنِ ابْنِ شِهَابٍ بِهَذَا الإِسْنَادِ وَلَمْ يَذْكُرْ بَلَغَنَا وَمَا بَعْدَهُ  

When a slave fulfils obligation of Allah and obligation of his master, he has two rewards for him. I narrated this to Ka'b, and Ka'b said: (Such a slave) has no accountability, nor has a poor believer.  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا أَدَّى الْعَبْدُ حَقَّ اللَّهِ وَحَقَّ مَوَالِيهِ كَانَ لَهُ أَجْرَانِ قَالَ فَحَدَّثْتُهَا كَعْبًا فَقَالَ كَعْبٌ لَيْسَ عَلَيْهِ حِسَابٌ وَلاَ عَلَى مُؤْمِنٍ مُزْهِدٍ  

The above hadith has been reported through another chain of transmitters on the authority of Abu Huraira.  

عَنِ الأَعْمَشِ بِهَذَا الإِسْنَادِ  

It is good for a slave that he worships Allah well, and serves his master (well). It is good for him.  

عَنْ رَسُولِ اللَّهِ ﷺ فَذَكَرَ أَحَادِيثَ مِنْهَا وَقَالَ قَالَ رَسُولُ اللَّهِ ﷺ نِعِمَّا لِلْمَمْلُوكِ أَنْ يُتَوَفَّى يُحْسِنُ عِبَادَةَ اللَّهِ وَصَحَابَةَ سَيِّدِهِ نِعِمَّا لَهُ