"I was fishing and I had a vessel with me in which I kept water, and I used seawater for ablution. I mentioned that to the Messenger of Allah and he said: 'Its water is a means of purification, and its dead meat is permissible.'"
Sunan Ibn Majah Book 1, Hadith 121
← Previous Hadith He heard Abu Hurairah say: "A man came to the Messenger of Allah and said: "O Messenger of Allah, we travel by sea and carry a small amount of water with us. If we use it for ablution, we will become thirsty. Can we perform ablution with seawater?' The Messenger of Allah said: 'Its water is a means of purification, ad its dead meat is permissible. (i.e. the fish found dead in the sea).'"

Sunan Ibn Majah Book 1, Hadith 120 Next Hadith → The Prophet was asked about seawater, and he said: "Its water is a means of purification, and its dead meat is permissible." (Hasan)

Another chain with similar wording.

Sunan Ibn Majah Book 1, Hadith 122