Musannaf Ibn Abi Shayba 28:941

Waki’ narrated to us, he said: Shu’bah narrated to us, from Qays ibn Muslim, from ‘Abd al-Rahman ibn Abi Layla: That ‘Umar ibn al-Khattab stipulated upon the people of al-Sawad the hospitality of a day and a night. So one of them would say: "Siyah, siyah," meaning a night.