The Prophet ﷺ said, "When a slave excels in the worship of his Lord and is sincere toward his master, he receives a double reward."
حدثنا يحيى عن عبيد الله ومحمد بن عبيد قال حدثنا عبيد الله عن نافع
عن النبي ﷺ العبد إذا أحسن عبادة ربه تبارك وتعالى ونصح لسيده كان له الأجر مرتين
Its isnad is sahih. Abu Dawud (4:508) narrated it via Malik from Nafi. Munziri said, Bukhari and Muslim also included it. Something similar in meaning has already passed as part of a hadith from the Musnad of Abu Bakr with a daif isnad, number 13.