Sayyiduna al-Mughirah bin Abi Burdah (may Allah have mercy on him) heard Sayyiduna Abu Hurayrah (may Allah be pleased with him) say: One day we were with the Messenger of Allah (peace and blessings be upon him) when a fisherman came and said: "O Messenger of Allah! We travel on the sea for the purpose of fishing, and one of us takes a water vessel with him, hoping that he will catch fish close by. Often that is how it happens, but sometimes he does not find any fish until he reaches such-and-such a place in the sea, which he had not expected to reach. Sometimes he has a wet dream, or his ablution is broken; then if he takes a ritual bath (ghusl) or performs ablution with this water, perhaps thirst will destroy one of us. What is your ruling about sea water? May we take a ritual bath or perform ablution with it when we fear this?" The Messenger of Allah (peace and blessings be upon him) said: "Take a ritual bath and perform ablution with its water, for its water is pure and its dead (animals) are lawful."
This same hadith is narrated from the Prophet (peace and blessings be upon him) by Sayyiduna Ali bin Abi Talib (may Allah be pleased with him), Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) and Sayyiduna Abdullah bin Amr (may Allah be pleased with them both).
The narrator whom al-Shafi'i meant by his words "in its chain there is someone I do not know" is Salamah bin Sa'id (as some named Sa'id bin Salamah), or al-Mughirah, or both of them; however, the one who established its chain is thiqah (trustworthy): Malik bin Anas placed it in the Muwatta, and Abu Dawud recorded it in the Sunan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 2