Ibn 'Umar Radiyallahu 'Anhu says. "Rasulullah Sallallahu 'Alayhi Wasallam kept his ring in his mubaarak hands (possession). Then it was kept by Abubakr Radiyallahu 'Anhu, then by 'Umar Radiyallahu 'Anhu. Thereafter by 'Uthmaan Radiyallahu 'Anhu. In his ('Uthmaan Radiyallahu 'Anhu's) time it fell in the Well of 'Arees. The inscription on this ring was 'Muhammadur Rasulullah"'.
English reference: Book 11, Hadith 89
Arabic reference: Book 12, Hadith 94