Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that a marriage takes place in the presence of four: 1. the guardian, 2. two witnesses, 3. the one marrying.
It has a supporting report from Ibn Abbas with a munqati' (broken) chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13814
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
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (the Prophet) (peace and blessings be upon him) said: "A marriage takes place in the presence of four: 1. the guardian, 2. the one marrying, 3. two upright witnesses."
This has also been narrated through another, weak route from Abu Salamah from Abu Hurayrah as marfu', and through another, weak route from Hisham bin Urwah, from his father, from Aishah (may Allah be pleased with her) as marfu'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13816