"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.'"
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.
حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا عِيسَى بْنُ يُونُسَ، حَدَّثَنَا الأَعْمَشُ، عَنْ مُسْلِمِ بْنِ صُبَيْحٍ، عَنْ مَسْرُوقٍ، عَنِ الْمُغِيرَةِ بْنِ شُعْبَةَ، قَالَ خَرَجَ النَّبِيُّ ـ صلى الله عليه وسلم ـ لِبَعْضِ حَاجَتِهِ فَلَمَّا رَجَعَ تَلَقَّيْتُهُ بِالإِدَاوَةِ فَصَبَبْتُ عَلَيْهِ فَغَسَلَ يَدَيْهِ ثُمَّ غَسَلَ وَجْهَهُ ثُمَّ ذَهَبَ يَغْسِلُ ذِرَاعَيْهِ فَضَاقَتِ الْجُبَّةُ فَأَخْرَجَهُمَا مِنْ تَحْتِ الْجُبَّةِ فَغَسَلَهُمَا وَمَسَحَ عَلَى خُفَّيْهِ ثُمَّ صَلَّى بِنَا .
"The Prophet went out to relieve himself and when he came back, I met him with a water skin and poured water for him. He washed his hands and his face, then he went to wash his forearms but his garment was too tight, so he brought his arms out from underneath his garment and washed them, then he wiped over his leather socks, then he led us in prayer."
"I brought a basin of water to the Prophet and he said: 'Pour it,' so I poured it and he washed his face and forearms, then he took fresh water and wiped his head, front and back, and then he washed his feet. He washed each part three times."
Abu Hurairah used to say: "The Messenger of Allah said: 'When anyone of you wakes up from sleep, he should not put his hand into the vessel until he has poured water on it two or three times, for none of you knows where his hand spent the night,'"
"The Messenger of Allah said: 'When anyone of oyu gets up from sleep and wants to perform ablution, he should not put his hand into the vessel he used for ablution until he has washed it, because he does not know where his hand spent the night or where he put it.'" [(One of the narrators) Abu Ishaq said: "What is correct is that it is narrated from Jabir, from Abu Hurairah."]