Friday Prayer is obligatory upon him, because he falls under the general purport of Allah’s saying:
“O you who believe! When the call is made for prayer on Friday, hasten to the remembrance of Allah and leave off trade.” [al-Jumu‘ah 62:9]
The hadith stating that Friday Prayer is not obligatory upon a slave is weak.
The reasoning that he is occupied with serving his master is even weaker, for there is no obedience to a created being if it entails disobedience to the Creator.
Some scholars said:
If his master gives him permission, then it is obligatory upon him, since he has no excuse [because the reason that prevented its obligation has ceased]. But if his master does not give him permission, then it is not obligatory upon him.
This is a balanced view, because the actual condition of a slave, when one considers it, is that of a weak person who is owned. He cannot say, “I am going to Friday Prayer, my master, whether you approve or not.” Thus, obligating him to do something he is unable to do would impose hardship. Allah has negated hardship for this ummah in the religion, saying:
“And He has not laid upon you in religion any hardship.” [al-Hajj 22:78]
This view occupies a middle position between the view of those who hold that Friday Prayer is unconditionally obligatory upon him and the view of those who hold that it is never obligatory upon him. Its basis is very strong. The hadith may also be understood accordingly: the Prophet’s ﷺ words, “an owned slave,” are not unrestricted; rather, the owned slave is one whom his owner keeps occupied. It may also be said that his ﷺ description, “owned,” indicates the effective reason for the ruling: he is property, so his master disposes of his time and keeps him occupied.
It is remarkable that those who held that Friday Prayer is not obligatory upon a slave nevertheless held that the congregational prayer is obligatory upon him. In my view, if the hadith of Tariq were authentic—that the Messenger ﷺ exempted the slave—then the non-obligation of the congregational prayer would apply all the more.