Musannaf Ibn Abi Shayba 3:2607

'Isa ibn Yunus narrated to us, from al-Awza'i, from Wasil, from Mujahid, that 'Umar ibn 'Abd al-'Aziz asked him about a people whose ship was wrecked, and the time for prayer arrived. He said: "Their leader shall be on their left, and they shall form a single row. Each man among them shall cover his private part with his left hand, without touching the private part."