من هو أول فدائي في الإسلام؟

It appears that the first person to sacrifice himself for the Prophet (peace and blessings be upon him) was Abu Bakr As-Siddiq (may Allah be pleased with him). This is based on what Al-Bukhari reported from Urwah bin Az-Zubair that Abdullah bin Amr bin Al-Aas said: While the Messenger of Allah (peace and blessings be upon him) was in the courtyard of the Kaaba, Uqbah bin Abi Mu'ayt approached and throttled the Prophet (peace and blessings be upon him) severely. Then Abu Bakr came and pushed Uqbah away from the Messenger of Allah (peace and blessings be upon him) and said: "Do you kill a man because he says: 'My Lord is Allah,' and he has brought you clear proofs from your Lord?!"

Al-Bazzar mentioned in a narration from Muhammad bin Ali bin Abi Talib, from his father, that Ali (may Allah be pleased with him) asked: "Who is the bravest of people?" They said: "You." Ali said: "Indeed, no one has ever challenged me to a duel except that I dealt justly with him. But it is Abu Bakr. I saw the Messenger of Allah (peace and blessings be upon him) being seized by the Quraysh; one would punch him, and another would tackle him, and they would say to him: 'Do you make the gods into one God?' By Allah, no one approached us except Abu Bakr, hitting one and pushing another, saying: 'Woe to you! Do you kill a man because he says: My Lord is Allah?'" Then Ali cried and said: "I ask you by Allah, is the believer from the people of Pharaoh better, or Abu Bakr?" The people remained silent. Then Ali said: "By Allah, an hour of Abu Bakr is better than him. That man concealed his faith, while this man declared his faith."

This incident occurred before Ali (may Allah be pleased with him) slept on the Prophet's bed during the Hijra (migration), because Abu Bakr's incident was before the Hijra.