Musannaf Ibn Abi Shayba 1:1391

Waki’ narrated to us, from Shu’bah, from Hisham bin Sa’d, from Zayd bin Aslam, from ‘Amr bin Sa’d al-Jari, who said: ‘Umar al-Jari came and called for napkins, then said: Perform ghusl (ritual washing) with seawater, for it is blessed.