Answered by Shaykh Abdurrahman ibn Yusuf Mangera
Many people have been telling me that wearing rings on the index finger and the thumb is haraam (prohibited)…is this statement true?
It is somewhat disliked for a man to wear a ring on the middle and index finger.
Ali radhiyallahu anhyu relates that the Messenger (upon him be peace) forbade me from wearing a ring in this finger and this finger. The narrator says he pointed to the middle and index finger (Sahih Muslim).
Imam Nawawi and ‘Allama Tibi has classified this to be a light reprehensibility [makruh tanzihan] and not “haram,” the reason being that these fingers are more frequently used hence it should be most preferably worn on the small finger otherwise on the “ring” finger. (See Takmila Fath al-Mulhim 4:139)
The thumb would probably come under the same jurisdiction as the index finger due to the above-mentioned reason.
And Allah knows best.
Abdurrahman ibn Yusuf