Is it impermissible to wear a ring on the index finger during the prayer?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

There is an authentic Hadeeth that indicates the prohibition of wearing a ring on the index and middle fingers. ‘Ali said, "The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, forbade me to wear a ring on these two fingers of mine"—he pointed to his middle finger and the one next to it." [Muslim]

Imaam An-Nawawi said:

"This Hadeeth was narrated outside of Muslim with another wording: "…he pointed to the index finger and the middle one." Muslims have collectively agreed that the Sunnah stance here is that a man wears the ring on the little finger (pinkie). It is disliked for a man to wear a ring on his middle finger and the one next to it based on the aforementioned Hadeeth, but it is not forbidden."

Hence, it is disliked to wear a ring on the index finger during the prayer or outside of it.

Allaah Knows best.