Ata, Tawus, (and, the narrator thinks, also) Mujahid and al-Hasan (may Allah have mercy on them) narrate from the Prophet (peace and blessings be upon him) that on the day of the Conquest of Makkah he (peace and blessings be upon him) said: "A believer is not to be killed in return for a disbeliever."
Imam al-Shafi'i (may Allah have mercy on him) said: It is commonly known among the scholars of the military expeditions (maghazi) that the Messenger of Allah (peace and blessings be upon him) said this in his sermon on the day of the Conquest, and it is narrated from the Prophet (peace and blessings be upon him) with a connected chain (musnad) through the hadith of Amr bin Shu'ayb and the hadith of Imran bin Husayn.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15911