Muwatta Malik, Book 54, Hadith 1809 (In-Book)

Malik related to me from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said, "When a slave gives good counsel to his master and worships Allah correctly, he has his reward twice over. "

حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ الْعَبْدُ إِذَا نَصَحَ لِسَيِّدِهِ وَأَحْسَنَ عِبَادَةَ اللَّهِ فَلَهُ أَجْرُهُ مَرَّتَيْنِ ‏" ‏ ‏.‏

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also referenced under: Book 54, Hadith 43 (USC-MSA)