Mishkat al-Masabih 13 Marriage كتاب النكاح 262

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»

‘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.)