Arabic text
عن ابن عمر رضي الله عنهما : أن رسول الله صلى الله عليه وسلم قال: «إن العَبْد إذا نَصَح لسيِّده، وأحسن عِبَادة الله، فله أجْرُه مَرَّتَين». عن أبي موسى الأشعري رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «المَمْلُوك الذي يُحْسِنُ عِبَادَةَ رَبِّهِ، وَيُؤَدِّي إلى سَيِّدِهِ الذي له عليه من الحَق، والنَّصيحة، والطَّاعة، له أجْرَان».
Translation
Ibn ‘Umar (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) said: 'If a slave serves his master sincerely and worships Allah properly, he will have a double reward.'" Abu Mūsa al-Ash‘ari (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) said: "A slave who worships his Lord properly and discharges his duties towards his master, and is sincere and obedient to him will get a double reward."
Explanation
If a slave carries out his duties towards his master, serving and obeying him in what is good and being sincere to him, and performs the obligations and shuns the prohibitions laid down by Allah, the Almighty, he shall have a double reward on the Day of Judgment. This is because he is under two obligations: 1. To fulfill the rights of his master, and if he does so, he will get a reward. 2. To obey his Lord, and if he does so, he will get a reward.
Source: HadeethEnc.com