The Prophet ﷺ used to wear the stone of his ring on the inner side of his palm.
أن النبي ﷺ جعل فص خاتمه في باطن كفه.
No references are available.
Bukhari related it in Kitab al-Libas (5876), and Muslim in Kitab al-Libas (2091), both from the hadith of Nafi, from Ibn Umar, who mentioned it in a longer context than this.