The default position when you stand up to commence prayer is putting hands on the side - sadl al-yadayn. This was the way Muslims in Medina, Makka, and later in cities such as Kufa, would have been praying, until the rise of the Umayyads in Syria.

The contrary position of qabd - folding hands - almost certainly got introduced by Mu’awiya and this Umayyad custom survived into Sunni orthodoxy, except for the Maliki madhhab.

Professor Yasin Dutton has written an interesting paper - Amal vs Hadith in Islamic Law - that tackles this topic where he says:

• The non-Sunni madhhab’s, however - the Ithna Asahri Shi’a, the Zaydi’s, the Isma’li’s and the Ibadi’s (Khawarij) - are all agreed, along with the majority of the Maliki’s, on sadl.

Shaykh Hamza Yusuf, a prominent contemporary Sunni scholar, has explained, in a video you can find online, how the act of folding hands, versus not, was seen as an indicator of political affiliation during those days such as the civil war between Imam ‘Ali and Mu’awiya. Clearly he sees sadl as the more authentic and well-evidenced position.

He makes the following key points:

• People who were on the side of Imam ‘Ali in the civil wars prayed with their hands by the side (sadl)
• Even the Khawarij who seceded from the side continued to pray like that.
• Later Shi’a Imams, for instance Imams al-Baqir and Ja’far al-Sadiq, prayed the same way, without folding.
• Basically the outward form of a person, practicing sadl or qabd, used to be an immediate and visual indicator of their political position with respect to Imam ‘Ali or Mu’awiya.

Hope this helps.