“ The Prophet (صلى الله عليه وسلم) said in an authentic hadeeth, collected by Imaam al-Bukhaaree in his Saheeh:
(( مَن قَتَلَ مُعاهَداً لَم يَرِحْ رَائِحَةَ الجَنَّةِ ، وَ إِنَّ رِيْحَهَا تُوجَدُ مِن مَسِيرَةِ أَرْبَعِينَ عاماً ))
"Whoever kills a person who is under an agreement or treaty (with the Muslims), he will not smell the fragrance of Paradise, even though it's fragrance can be found from the distance of 40 years away." 
And in another narration:
(( حَرَّمَ اللهُ عَليهِ الجَنَّة ))
"Allaah has forbidden for him Jannah."
And Shaykhul-Islaam Ibn Taymiyyah (رحمه الله تعالى) said that the blood of the Kaafir is protected by one of four things:
1) he accepts Islaam
2) he is a non-muslim living in the lands of the Muslims (dhimmee)
3) he is from a country which has a treaty with the Muslims
4) he is given Amn, or an assurance of safety and protection by a Muslim
And the Prophet (صلى الله عليه وسلم) said:
(( ذِمَّةُ المسلِمينَ وَاحِدَةٌ ، فَمَن أَخْفَرَ مسلِماً فَعَليهِ لَعنَةُ اللهِ وَ المَلاَئِكَةِ وَ النَّاسِ أَجمَعِين ))
"The assurance of protection of the Muslims is one, so whoever betrays the assurance of protection and safety of another Muslim, then upon him is the curse of Allaah, and (the curse) of the Angels, and (the curse) of all the people."
al-Haafidh Ibn Hajr (رحمه الله) explained: "His saying 'The assurance of protection of the Muslims is one', means that their giving an assurance of safety and security is correct (and binding). So if one of them gives Amn to a Kaafir, then it is haraam upon anyone else to harm him…"
So – as Shaykh Taariq as-Subay’ee explains - if even one of the Muslims gives Amn to a non-Muslim, even if that Muslim is a man or a woman, a free man or a slave, or a person is given Amn by the authorities of a Muslim land through diplomatic agreements or by giving him a visa to enter that land – that person has an assurance of safety and protection from every Muslim. It is totally haraam for any Muslim to harm him. And the person, - the Muslim -, who does, falls under the curse of Allaah, the Angels, and all of the people.
But yet we find many of these people who blow up housing complexes, attack embassies, and kill those who have been given Amn by the country that they live and work in, they call this Jihaad!? They make shedding the blood of those who have been promised safety and security lawful, and they make breaching agreements, deceit, and treachery lawful! And they call this Islaam and they call this Jihaad?!!
Rather this is the way of the Khawaarij (and those who follow their way from the groups and parties). ” 
 Al-Bukhaaree. See Saheeh al-Jaami' # 6457
 Originally from Khutbah notes for a khutbah delivered June, 2004 in Masjid al-‘Uthmaan (Kuwait City) under the auspices of the Ministry of Awqaaf (Kuwait), with guidance and advice from Shaykh Taariq as-Subay’ee. Edited for the events in Libya (11 Sept., 2012) as a reminder.