Qatadah narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that a marriage is in the presence of four: 1. the one marrying, 2. the guardian, 3. two witnesses.
This chain is sahih, except that Qatadah did not meet Ibn Abbas. It has also been narrated through another, weak route from Ibn Abbas as marfu' (attributed to the Prophet), but what is well known from him is mawquf (his own statement); and it has also been narrated from the Prophet (peace and blessings be upon him) through another route.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13815