Hadith about having 2 witnesses for marriage is weak
The Hadith from which the non-followers deduce the necessity of having 2 witnesses for marriage is not rigorously authenticated with respect to its chain of narration, but is weakly authenticated (Dha‘eef). The necessity of having 2 witnesses is deduced from the following Hadith:
Translation: Marriage is valid only with the permission of the guardian (Wali) in the presence of 2 witnesses.
Imam Daraqutni has mentioned this Hadith in his Sunan. In the chain of narration of Imam Daraqutni, there is ‘Abdullah bin Muhriz about whom Imam Ibn Hajr ‘Asqalani has written:
Translation: ‘Abdullah bin Muhriz is a very unreliable narrator. (Lisan Ul Meezaan Li Ibn Hajr ‘Asqalani)
A very famous and authoritative scholar of the non-followers Siddiq bin Hasan Qanuji writes in Ar Rauzatun Nadiyya after mentioning this Hadith:
Translation: In the chain of narration of this Hadith, there is ‘Abdullah bin Muhriz who is a Forsaken Narrator (Matruk Rawi, from whom narration of Hadith has been abandoned). (Ar Rauzatun Nadiyya, Kitab Un Nikah, Vol. 2, Pg. No. 22)
In another chain of narration of this Hadith, there is ‘Adi bin Fadhl about whom Imam Baihaqui (May Allah shower His mercy on him) writes:
Translation: ‘Adi bin Fadhl has narrated this although he is unreliable. With a rigorously authenticated chain of narration, this is a Mauquf Hadith (A Hadith whose chain of narration goes back only to aTaba‘ee). (Sunan Kubra Lil Baihaqui, Kitab Un Nikah, Babu La Nikaha Illa Bi Waliyyin Murshidin, Vol. 7, Pg. No. 202, Hadith No. 13494)
Imam Ibn Hajr ‘Asqalani (May Allah shower His mercy on him) has written in At Talkhees Ul Habeer about the same narrator:
Translation: ‘Adi Bin Fadhl is an unreliable narrator. (At Talkhees Ul Habeer Li Ibn Hajr Il ‘Asqalani Kitab Un Nah ‘Anil Khitbati ‘Alal Khitbah Babul Auliyai Wa Ahkaamihim)