Al-Adab Al-Mufrad, Book 9, Hadith 202 (English) ⓘ
'Abdullah ibn 'Umar reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "When the slave advises his master and is assiduous in the worship of his Lord, he receives a double reward."
حَدَّثَنَا إِسْمَاعِيلُ قَالَ: حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللهِ صلى الله عليه وسلم قَالَ: إِنَّ الْعَبْدَ إِذَا نَصَحَ لِسَيِّدِهِ، وَأَحْسَنَ عِبَادَةَ رَبِّهِ، لَهُ أَجْرُهُ مَرَّتَيْنِ.
also referenced under: Hadith 202 (Reference) & Book 9, Hadith 47 (In-Book)