ENOfficial
‘Abdallah b. ‘Umar reported God’s Messenger as saying, “When a slave acts sincerely towards his master and worships God well, he has a double reward.” (Bukhari and Muslim.)
Source: sunnah.com
Other translations
Arabic
وَعَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «إِنَّ الْعَبْدَ إِذَا نَصَحَ لِسَيِّدِهِ وَأَحْسَنَ عِبَادَةَ اللَّهِ فَلَهُ أَجْرُهُ مرَّتَيْنِ»
sunnah.com