Sunan Abi Dawud Book 15, Hadith 13
The Prophet (ﷺ) said: No one should sail on the sea except the one who is going to perform hajj or umrah, or the one who is fighting in Allah's path for under the sea there is a fire, and under the fire there is a sea.
A woman called Umm Khallad came to the Prophet (ﷺ) while she was veiled. She was searching for her son who had been killed (in the battle) Some of the Companions of the Prophet (ﷺ) said to her: You have come here asking for your son while veiling your face? She said: If I am afflicted with the loss of my son, I shall not suffer the loss of my modesty. The Messenger of Allah (ﷺ) said: You will get the reward of two martyrs for your son. She asked: Why is that so, Messenger of Allah? He replied: Because the people of the Book have killed him.
Anas bin Malik (may Allaah be pleased with him) said “Umm Haram, daughter of Milhan, sister of Umm Sulaim, narrated to me that the Apostle of Allaah(ﷺ) took a mid day nap with them. He then awoke laughing. She said “I asked the Apostle of Allaah(ﷺ), what made you laugh?” He replied “I saw some people who ere sailing in the midst of the sea like kings on thrones. She said “I said the Apostle of Allaah(ﷺ) beseech Allaah that He may put me among them. He replied “You will be among them.” She said “He then slept and awoke laughing. She said “I asked the Apostle of Allaah(ﷺ), what made you laugh? He replied as he said in the first reply. She said “I said the Apostle of Allaah(ﷺ) beseech Allaah that HE may put me amongst them. He replied “You will be among the first. Then ‘Ubadah bin Al Samit married her and sailed on the sea on an expedition and took her with him. When he returned, a riding beast was brought near her to ride, but it threw her down. Her neck was broken and she died.