Abu Hurayrah (may Allah be pleased with him) narrates: I saw seventy of the students of the Suffah, and not one of them had an upper sheet; there was only a striped sheet or a coarse blanket, which they had tied around their necks. Some of these reached half the shin and some reached the ankles, and they would gather the cloth together with their hand so that their private parts would not be exposed.
Imam al-Bukhari (may Allah have mercy on him) recorded this same hadith in his Sahih through Yusuf bin Isa.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3303