The Islamic Personality by Sheikh Taqīuddīn An-Nabahānī

The contract of hiring or employment which came upon the benefit of work and the benefit of the person is the one which relates to the employee. The employee is one who hires himself. The Shar’a has permitted the hiring of the human being for a benefit resulting from him like service and the like or the benefit resulting from his work like painting, dyeing, engineering and the like. In order to contract the hiring, the fitness of the two contractors is a condition for its contracting such that each of the two should be sane and possessing discretion. The hiring of the insane is not contracted, nor the hiring of an immature child. If the hiring contracts, the consent of the two contractors is a condition of its validity, and that which is contracted upon— which is the benefit—be known in a way that prevents dispute. This knowledge of the service, in relation to the employee, at times is by explaining the period, at times by limiting the benefit or describing the required work in a detailed description and specifying what the employee will do or specifying how he will work. Accordingly, hiring the compelled person is not valid nor is the hiring of an unknown benefit valid.