Sahih al-Bukhari, Book 49, Hadith 34 (In-Book) ⓘ
Narrated `Abdullah: The Prophet said, "If a slave serves his Saiyid (i.e. master) sincerely and worships his Lord (Allah) perfectly, he will get a double reward."
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 " .
also referenced under: Hadith 2550 (DarusSalam) & Volume 3, Book 46, Hadith 726 (USC-MSA)