كتاب الأيمان
27  
The Book of Oaths
(11)
Chapter: The reward of a slave who is sincere towards his master and worships Allah properly
(11)
باب ثَوَابِ الْعَبْدِ وَأَجْرِهِ إِذَا نَصَحَ لِسَيِّدِهِ وَأَحْسَنَ عِبَادَةَ اللَّهِ ‏‏
Sahih Muslim 1664 a

Ibn Umar reported Allah's Messenger (ﷺ) as saying:

When a slave looks to the welfare of his master and worships Allah well, he has two rewards for him.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ إِنَّ الْعَبْدَ إِذَا نَصَحَ لِسَيِّدِهِ وَأَحْسَنَ عِبَادَةَ اللَّهِ فَلَهُ أَجْرُهُ مَرَّتَيْنِ ‏"‏ ‏.‏
Reference : Sahih Muslim 1664a
In-book reference : Book 27, Hadith 65
USC-MSA web (English) reference : Book 15, Hadith 4097
  (deprecated numbering scheme)