The sandals of the Messenger of Allah ﷺ had two straps, each with a doubled thong.
كان لنعل رسول الله ﷺ قبالان، مثني شراكهما.
Ibn Majah (3614), Tirmidhi in Shamail (76), and Bayhaqi in Shuab (5860) all narrated it through the hadith of Waki, from Sufyan (that is, Thawri), from Khalid Haddan, from Abdullah bin Harith, from Ibn Abbas, and he related it. Its isnad is sahih. Hafiz strengthened its chain in Fath al-Bari (10/312).