Sometime before the Treaty of Hudaybiyyah, the Prophet ﷺ sent 300 men under Abu Ubaydah Amir ibn al-Jarrah RA to intercept a caravan of the Quraysh, most likely on its way to Syria. The Quraysh escaped them. As the Muslims gave chase, their supplies ran dangerously low. Abu Ubaydah collected all the food in the army and rationed it, until each man was getting a single date a day. When even that ran out, the companions were forced to eat khabat, the dried, withered leaves of thorn bushes. The expedition is named after these leaves.
Eventually they reached the shore at Yanbu, not far from Madinah, and found an enormous whale washed up on the beach. Thirteen or fourteen of the companions could sit together in the socket of its eye. They set one of its bones upright, and Abu Ubaydah rode his largest camel underneath it without lowering his head. The army camped there for a month, living off the whale.
They returned to Madinah with the leftover meat and asked the Prophet ﷺ whether it had been allowed to eat an animal they had found already dead. He asked whether any of the meat was left, had it brought to him, and ate from it himself.
Outcome: The army survived near-starvation on the whale; the Prophet ﷺ confirmed seafood as halal by eating from it himself.
A foundational hadith for the ruling, held by all madhhabs except the Hanafi, that all creatures of the sea are lawful to eat without need for slaughter, whether caught or found already dead.
Other events recorded in 6 AH. Their order within the year is not established; use the storyline above for the sequence.
We ran out of food entirely, so we would put the date seeds in our mouths and suck on them, and drink water with them for some taste; we were forced to eat khabat (the dried withered leaves of thorn bushes) just to survive.
We found a massive whale washed ashore at Yanbu, and thirteen or fourteen of us sat together in the socket of its eye. Abu Ubaydah picked out one of its bones, planted it upright on the beach, and rode his largest camel right underneath it without lowering his head.
Do you have any meat of it left? ... Bring it to me.
Asked of the returning sahaba, then he ﷺ ate from the whale meat they had kept, to show it was halal.
Two dead animals are allowed for us: locusts and the whale.
"Sayd al-bahr" (صيد البحر) in the verse refers to what you catch, and "ta'am al-bahr" (طعام البحر) is what is dead and washes up to you.
“It is lawful for you to hunt and eat seafood, as a provision for you and for travelers.”
Cited by the Prophet ﷺ's teaching to explain the permissibility of seafood.