Sahih Muslim · Hadith 4408
The Book of Oaths - كتاب الأيمان
حدثنا يحيى بن يحيى، قال قرأت على مالك عن نافع، عن ابن عمر، أن رسول الله صلى الله عليه وسلم قال " إن العبد إذا نصح لسيده وأحسن عبادة الله فله أجره مرتين " .
Ibn Umar reported Allah's Messenger (may peace be upon him) as saying: When a slave looks to the welfare of his master and worships Allah well, he has two rewards for him.
Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).