The contract comes into force when the parties utter the word “ijārah,” the word “kirāʾ,” or any other expression that conveys the same meaning.

Shaykh Taqī al-Dīn said: The sound position is that once the two parties understand one another, the contract is valid no matter what wording they employ, provided the expressions used are known to both sides as signifying their intended transaction. This applies to every type of contract, for the Lawgiver has not fixed any specific wording for contracts; rather, He left their formulation unrestricted.

Shaykh ʿAbd al-Raḥmān al-Saʿdī explained that contracts fall into three categories:

1. Binding contracts, which themselves are of two kinds:
   a. Those that become effective the moment they are concluded and allow no option of termination—such as an endowment (waqf), marriage, and the like.
   b. Those that are binding in principle, yet the Lawgiver has granted within them the option of the session (khiyār al-majlis) and the option stipulated by condition (khiyār al-sharṭ); examples include sales, leases (ijārah), settlements (ṣulḥ), and similar transactions.

2. Non-binding contracts for both parties, so either side may annul them at will. These include agency (wakālah), guardianship (wilāyah), jiʿālah (a promise of reward for accomplishing a task), and partnerships.

3. Contracts that are binding on one party but optional for the other. The criterion here is that the right resides with one side over the other, as in the case of a pledgor (rāhin), guarantor (ḍāmin), or surety (kāfil): the obligation is binding upon these individuals, yet optional for the pledgee, the creditor whose debt is guaranteed, and the beneficiary of the surety. And Allah knows best.