On what basis do Shi'a Muslims claim that a marriage is valid even if done in the absence of witnesses?

AsSalam Aalykum. I am aware of the fact that a Sunni marriage will not be considered valid if there are no witnesses for the marriage. As far as I know, there are ahadith which stress on the necessity of this condition. I haven't found verses in the Qur'an yet about witnesses in marriages but I found ahadith on [IslamQA](https://islamqa.info/en/answers/113868/adulthood-is-a-condition-of-being-a-witness-to-marriage) with regards to this matter: > In order for a marriage to be valid, it is stipulated that it be > witnessed by two Muslim witnesses of good character, because the > Prophet (peace and blessings of Allaah be upon him) said: “There is no > marriage except with a wali (guardian) and two witnesses of good > character. Narrated by al-Bayhaqi from the hadeeth of ‘Imraan and > ‘Aa’ishah; classed as saheeh by al-Albaani in Saheeh al-Jaami’ no. > 7557. Shi'a Muslims, on the other hand, [claim that](https://www.al-islam.org/marriage-according-five-schools-islamic-law-allamah-muhammad-jawad-maghniyyah/marriage-contract-and#fref_5311d421_2) it is *mustababb* for the witnesses to be present but not *wajib*. I believe that any Islamic ruling should be firmly backed by the Qur'an and the sayings of Prophet Muhammad (PBUH). Relevant statement from the link above: > The Imamiyyah do not consider the presence of witnesses as wajib but > only mustababb. The following is stated regarding [marriage](https://www.sistani.org/english/book/46/2062/) witnesses on Al-Sayyid Ali Al-Husseini Al-Sistani's website: > It is permissible for the couple to recite the formula of marriage > agreement by themselves or by appointing representatives who will > recite it on their behalf. There is no condition for the presence of > witnesses during the solemnization of the marriage, just as the > presence of a cleric is not a condition for the validity of the > marriage. On a website called Alulbayt Foundation, the following answer (and question) is given for marriages: > Q14) Can a permanent marriage be made secretly without the permission > of parents of bride and without the witness being called? > > A14) **Presence of two witnesses is not necessary when a marriage > contract is being read.** And a girl who is not independent in her > living affairs should obtain permission of her father or paternal > grandfather for marriage. And if she is independent, she should obtain > her guardian’s permission as a measure of obligatory precaution. On Wikipedia, for the topic of [Islamic marriage contract](https://en.wikipedia.org/wiki/Islamic_marriage_contract) , the following has been stated about witnesses for Shi'a marriages: > In Shia Islam, witnesses to a marriage are deemed necessary, but in > case are not available then the two parties may conduct the nikah > between themselves. Which Qur'anic verses and/or ahadith support the Shi'a ruling on the presence/absence of witnesses for marriages?