عن أبي هريرة رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «للعبد المملوك المصلح أجران»، والذي نفس أبي هريرة بيده لولا الجهاد في سبيل الله والحج، وبِرُّ أمي، لأحببت أن أموت وأنا مملوك. [صحيح] - [متفق عليه] المزيــد ...
Abu Hurayrah (may Allah be pleased with him) narrated that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "The dutiful slave will be given a double reward." By the One in Whose hand the soul of Abu Hurayrah is! Had it not been for Jihad in the cause of Allah, Hajj, and looking after my mother, I would have liked to die as a slave. Sahih/Authentic. - [Al-Bukhari and Muslim]
If the slave fulfills his master's rights upon him by obeying his orders (which are legal and reasonable) as well as fulfills the rights of Allah, the Almighty, upon him by performing his religious duties and staying away from the prohibitions, then Allah will give him double reward on the Day of Resurrection. The first reward is for fulfilling his master's rights, and the second is for fulfilling Allah's rights upon him.
Abu Hurayrah (may Allah be pleased with him) after narrating this Hadīth, swore by Allah that had it not been for his desire to perform Jihad in the cause of Allah, Hajj, and to look after his old mother, he would have wished to be a slave in order to receive the promised double reward.
A slave cannot go out to fight in jihad except with his master's permission, who may deny him permission because he needs his service or fears that he might be killed.
A slave also cannot set out to perform Hajj without the permission of his master, who might refuse because he needs him.
The third reason that prevented him from wishing to be a slave is his dutifulness to his mother by obeying and looking after her. If he had been a slave, obedience to his master would take precedence over obedience to his mother, and his master's rights upon him would be higher in priority than his mother's rights. The labor and benefit of the slave is owned by his master, who has full authority over him. This possibly may hinder him from fulfilling the rights of his mother and being obedient to her.