It is narrated from al-Hakam that Sayyiduna Sa'd (may Allah be pleased with him) led the people in prayer while his hands were inside his sleeves.
Abu Ubayd (may Allah have mercy on him) says: al-mustuqah means a garment with long sleeves.
Ibrahim al-Nakha'i (may Allah have mercy on him) says that people used to pray in their long-sleeved garments, cloaks and large shawls (a green-coloured shawl usually worn by shaykhs), and would not take their hands out.
This report is mursal. A hadith with a connected chain has also been narrated on this, but its chain has some weakness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2674