Hadith: A man came to the Messenger of Allah (may Allah's peace and blessings be upon him) and said: @O Messenger of Allah, what if a man comes to me wanting to take my money?* He said: "Then do not give him your money." The man said: What if he fights with me? He said: "Then fight with him." The man said: What if he kills me? He said: "Then, you would be a martyr." The man said: What if I kill him? He said: "He would be in Hellfire."

عَنْ أَبِي هُرَيْرَةَ رَضيَ اللهُ عنه قَالَ:
جَاءَ رَجُلٌ إِلَى رَسُولِ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ، فَقَالَ: يَا رَسُولَ اللهِ، أَرَأَيْتَ إِنْ جَاءَ رَجُلٌ يُرِيدُ أَخْذَ مَالِي؟ قَالَ: «فَلَا تُعْطِهِ مَالَكَ» قَالَ: أَرَأَيْتَ إِنْ قَاتَلَنِي؟ قَالَ: «قَاتِلْهُ» قَالَ: أَرَأَيْتَ إِنْ قَتَلَنِي؟ قَالَ: «فَأَنْتَ شَهِيدٌ»، قَالَ: أَرَأَيْتَ إِنْ قَتَلْتُهُ؟ قَالَ: «هُوَ فِي النَّارِ».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 140]

Abu Hurayrah (may Allah be pleased with him) reported:
A man came to the Messenger of Allah (may Allah's peace and blessings be upon him) and said: O Messenger of Allah, what if a man comes to me wanting to take my money? He said: "Then do not give him your money." The man said: What if he fights with me? He said: "Then fight with him." The man said: What if he kills me? He said: "Then, you would be a martyr." The man said: What if I kill him? He said: "He would be in Hellfire."
[Authentic hadith] - [Narrated by Muslim] - [Sahih Muslim - 140]

Explanation

A man came to the Prophet (may Allah's peace and blessings be upon him) and said: O Messenger of Allah, what if a man comes to me wanting to take my money? He said: You are not required to submit to him and give him your money. He said: What if he fights with me? He said: You are permitted to fight him. He said: What if he kills me? He said: Then you would be a martyr. He said: What if I kill him? He said: He deserves to be punished in Hellfire on the Day of Judgment.

Benefits from the Hadith

1. An-Nawawi said: Defending one's womenfolk is obligatory without disagreement. As for defending oneself to the point of killing, there is a difference of opinion in our school and others; whereas, defending one's wealth is permissible but not obligatory.
2. The Hadīth is evidence that knowledge precedes action, as this Companion asked the Messenger of Allah (may Allah's peace and blessings be upon him) about what was required of him before acting.
3. One should follow a gradual approach in repelling an aggressor—by advising him or calling for help—before resorting to fighting him. But if it starts fighting, the aim should be to repel him, not to kill him.
4. The blood, wealth, and honor of a Muslim are inviolable.
5. An-Nawawi said: Know that martyrdom is of three categories. The first is the one killed in battle against the disbelievers due to one of the causes of the fighting. This person has the rulings of martyrs both in the Hereafter and in worldly terms, meaning he is not to be washed or prayed over. The second is considered a martyr in terms of reward only, not in worldly rulings—such as the one who dies of a stomach illness, the one who dies in a plague, the one who dies under a collapsed building, the one killed while defending his wealth, and others whom authentic Hadīths have referred to as martyrs. Such a person is washed and prayed over, yet he receives the reward of martyrs in the Hereafter, though not necessarily equal to the reward of the first category. The third is someone who, for example, stole from the war booty or committed similar acts, and for whom narrations have come denying him the title of martyr, even if he was killed in battle against the disbelievers. This person has the rulings of martyrs in the worldly sense—so he is not washed or prayed over—but he does not receive the full reward of martyrs in the Hereafter.