Musannaf Ibn Abi Shayba 12:199

Abu Bakr narrated to us, he said: Hatim ibn Isma'il narrated to us, from Humayd ibn Sakhr, from Muhammad ibn Ka'b al-Qurazi, from Ibn Abbas regarding His saying: "Lawful to you is the game of the sea and its food," he said: What the sea casts upon its back dead.