Musannaf Ibn Abi Shayba 1:1379

'Abd al-Rahim narrated to us, from 'Ubayd Allah ibn 'Umar, from 'Amr ibn Dinar, from Abu al-Tufayl, who said: Abu Bakr al-Siddiq was asked, "Does one perform ablution with seawater?" He said, "It is the purifier, its water is lawful, and its dead [creatures] are lawful."