Ruling 2382
To conclude a marriage contract, whether that be for a permanent marriage or a temporary one, a formula (ṣīghah) must be said; the mere consent of the man and the woman is not sufficient, nor is a written contract, based on obligatory precaution (al‑iḥtiyāṭ al‑wājib). The man and the woman can say the formula themselves, or they can appoint an agent (wakīl) to say it on their behalf.
-Ayatullah Sistani, English Islamic Laws 4th Edition Page 421