Sahih al-Bukhari 2550

Arabic

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ، حَدَّثَنِي نَافِعٌ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا نَصَحَ الْعَبْدُ سَيِّدَهُ، وَأَحْسَنَ عِبَادَةَ رَبِّهِ، كَانَ لَهُ أَجْرُهُ مَرَّتَيْنِ ‏"‏‏.‏

English translation

Narrated `Abdullah:The Prophet (ﷺ) said, "If a slave serves his Saiyid (i.e. master) sincerely and worships his Lord (Allah) perfectly, he will get a double reward

Reference: Sahih al-Bukhari 2550, Book of Manumission of Slaves. English translation by Dr. Muhammad Muhsin Khan.