Abu Hurayrah radiallahu anhu says that the shoes of Sayyidina Rasulullah sallallahu alaihe wasallam had two straps. In the same manner the shoes of Abu Bakr and Umar radiallahu anhu had two straps on them. Sayyidina 'Uthman radiallahu anhu began the use of one strap.
English reference: Book 10, Hadith 81
Arabic reference: Book 11, Hadith 86