Musnad Abu Ya'la al-Mawsili 243:87

On the authority of Abu Musa, from the Prophet (peace and blessings of Allah be upon him), he said: "For the slave who performs the worship of his Lord well, and fulfills for his master that which is due upon him of right, sincerity, and obedience, there are two rewards: the reward for having performed the worship of his Lord well, and the reward for having fulfilled that which is due to his master."