According to al-Farabi, what is the relation of philosophy and religion?

Al-Farabi is very interested in the role that religion plays within philosophy. Religion, according to al-Farabi, is a means through which people can achieve happiness and peace. It is through religion that individuals transcend day-to-day concerns (though religious belief also affects these concerns) and focus on self-improvement. In this belief, al-Farabi is influenced by Aristotle's concept of ethics. The moral dimension of religion encourages people to do good things, and can be a moderating force. Al-Farabi also saw religion as a means that rulers of a society could use to encourage desirable actions on the part of their subjects:

Religion is opinions and actions, determined and restricted with stipulations and prescribed for a community by their first ruler, who seeks to obtain through their practicing it a specific purpose with respect to them or by means of them...

A good ruler, according to al-Farabi, is one not unlike Plato's ideal of a "philosopher king." This ruler would understand truth, and would use the state to promote it. Religion was a very important tool in providing a sort of common ethic that all in the state could embrace. So al-Farabi tended to think of religion in somewhat utilitarian, almost sociological terms. It was important, indeed indispensable, but not so much as a system of values in itself. Religion was a means to achieve important ends, both as a society and individually.
https://plato.stanford.edu/entries/al-farabi-soc-rel/