The faithful and diligent slave will receive a double reward

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said that the slave who faithfully serves his master and fulfills the rights of his Lord will have a double reward. He receives a reward for fulfilling Allah's right upon him in terms of worship and another reward for fulfilling his master's right upon him in terms of service.
Abu Hurayrah then added that were it not for the fact that jihad is not obligatory on the slave (and there is a great reward for performing Jihad) and the fact that he used to provide for his mother and take care of her, he would have preferred to die as a slave to receive the double reward.