Shama-il Tirmidhi Chapter 10, Hadith Number 11 (81).

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 radiallahu anhu began the use of one strap.

Commentary.
Sayyidina radiallahu anhu may have adopted the use of one strap so that it may not be assumed that two straps are strictly necessary.