If a slave is sincere

‘Abd Allah b. ‘Umar reported the Messenger of Allah ﷺ as saying:
when a slave acts sincerely towards his master and worship Allah well, he will have a double reward.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ الْقَعْنَبِيُّ، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ " إِنَّ الْعَبْدَ إِذَا نَصَحَ لِسَيِّدِهِ وَأَحْسَنَ عِبَادَةَ اللَّهِ فَلَهُ أَجْرُهُ مَرَّتَيْنِ " .