الْبَيْعُ يَنْعَقِدُ بِالْإِيجَابِ وَالْقَبُولِ إِذَا كَانَا بِلَفْظِ الْمَاضِي.
Translation: A sale is concluded by offer and acceptance when both are expressed in the past tense.
Ijab is the legally operative opening declaration; qabul is the answering declaration. Together they form the contract.
The past-tense forms “bi‘tu” (I have sold) and “ishtaraytu” (I have bought) are used performatively: by uttering them as contractual declarations, the parties form a transaction now rather than merely narrating a past event.