Sahih Bukhari, Chapter: Prayers (Salat)

I saw seventy of As-Suffa men and none of them had a Rida' (a garment covering the upper part of the body). They had either Izars (only) or sheets which they tied round their necks. Some of these sheets reached the middle of their legs and some reached their heels and they used to gather them with their hands lest their private parts should become naked.