The Prophet's ﷺ sandal had two straps.
حدثنا أبو بكر بن أبي شيبة حدثنا يزيد بن هارون عن همام عن قتادة
كان لنعل النبي ﷺ قبالان
No references are available.
Bukhari narrated it in "Clothing," in the chapter "Two straps on a sandal, and whoever considers one wide strap sufficient," Hadith 5857, from the hadith of Hammam, through him, and it is also in Musannaf: 8/231.