Sahih Muslim , Book 15, Hadith 4097 (USC-MSA) ⓘ
Ibn Umar reported Allah's Messenger 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 " .
also referenced under: Book 27, Hadith 66 (In-Book) & Hadith 1664 (Reference)