“None of you should walk in a single sandal; let him either remove them both or wear them both.”

Narrated by al-Bukhārī (no. 5856, and this is his wording), and Muslim (no. 2097), from the narration of Abū Hurayrah — may Allah be pleased with him.

Brief Explanation of the Hadith

From the perfection of the Sharī‘ah is that it did not limit itself to refining the inward state, but also gave attention to the outward. It laid down detailed etiquettes that preserve a person’s dignity and composure in dress and manner of walking. Among these is the Prophet’s — may Allah’s peace and blessings be upon him — prohibition against walking in a single sandal…
And the wisdom behind this is clear: wearing a single sandal is the manner of the devil, as mentioned in another ḥadīth: “Indeed, the devil walks in a single sandal.”
It also carries an appearance of distortion and disfigurement, and it contradicts the manner of people of dignity and honor. One who sees a person in such a state may attribute it to unsound judgment or lack of propriety. Moreover, it may lead to stumbling and falling, in addition to being a form of giving preference and honor to one foot over the other. The Sharīʿah, however, is founded upon justice and balance.
Most scholars have understood the prohibition here to indicate dislike (karāhah), and such dislike is waived in cases of need or excuse.
The ḥadīth also points to Islam’s concern with refining the conduct of the Muslim and regulating his outward appearance, so that he may present himself in the finest form of balance and dignity — both outwardly and inwardly.