Why did the Prophet, sallallaahu ‘alayhi wa sallam, forbid wearing a ring on the index and middle fingers? What is the wisdom behind this forbiddance?

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

The Prophet, sallallaahu ‘alayhi wa sallam, forbade men in particular to wear rings in the index and middle fingers. The Prophet, sallallaahu ‘alayhi wa sallam, wore a ring in the little finger of his left hand. This is because, being at this position, the ring is kept away from filths that could be touched by the hand and also does not hinder the hand from doing something or catching something.

So it is recommended for men to wear the ring in the little finger of the left hand. It was narrated on the authority of Anas that he said, "The Prophet, sallallaahu ‘alayhi wa sallam, wore a ring in this – pointing to the little finger of his left hand." [Muslim]

As for women, they are permitted to wear rings on all their fingers and toes.

Commenting on Saheeh Muslim, An-Nawawi said, "There is a consensus among the Muslims that it is recommended for men to wear the ring on the little finger of the left hand.  As for women, they are permitted to wear rings on all their fingers."

They also said that the wisdom behind this ruling is that, unlike the index finger, the little finger is farther from filths that could be touched by the hand and it also does not hinder the hand from doing or catching something.

We alert the noble questioner that a Muslim, in principle, has to surrender to the rulings of the Sharee'ah in all cases, as much as he can. He also has to firmly believe that there is wisdom behind these rulings, whether it is known or unknown. Meanwhile, a Muslim is allowed to ask about such rationales and use his mind to discover them, because when one comes to know such rationales, his heart becomes more assured and his faith increases.

Allah knows best.