Al-Matalib al-'Aliyah 2883 — Prohibition of harming anyone who says

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When one of you points his spear at a (hostile) man, and it has come right up to the hollow of his throat (his jugular), and then that man says 'La ilaha illallah' (there is no god worthy of worship except Allah), then (the attacker) should immediately lift his spear away from him." The narrator said that Abu Ubaydah (may Allah have mercy on him) said: "So Allah, the Blessed and Exalted, has made this word (of tawhid) a source of safety for the Muslim and a means of protecting and safeguarding his blood, and (likewise) He has made the jizyah a source of safety for the (protected) disbeliever and a means of safeguarding his life."
Takhrij and grading: This hadith is narrated in Musnad Ahmad (2883); its chain is broken, because the hearing of Abu Ubaydah bin Abdullah bin Mas'ud from his father is not established; however, its meaning regarding the sanctity of the blood of one who professes the word of tawhid is established and supported by other sahih and authentic hadith.