Sunan Ibn Majah, Volume 4, Book 32, Hadith 3648 (DarusSalam) ⓘ
It was narrated that ‘Ali said: “The Messenger of Allah forbade me to wear a ring on this and on this,” meaning the little finger and the thumb.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ إِدْرِيسَ، عَنْ عَاصِمٍ، عَنْ أَبِي بُرْدَةَ، عَنْ عَلِيٍّ، قَالَ نَهَانِي رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ أَنْ أَتَخَتَّمَ فِي هَذِهِ وَفِي هَذِهِ يَعْنِي الْخِنْصَرَ وَالإِبْهَامَ .
also referenced under: Hadith 3648 (Reference) & Book 32, Hadith 3779 (In-Book)