Mustadrak al-Hakim — Hadith 3240

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) concerning this saying of Allah Almighty: "And if he was from a people hostile to you while he was a believer, then (the penalty is) the freeing of a believing slave" that he said: "It used to happen that a man would come to the Messenger of Allah (peace and blessings be upon him) and accept Islam, then return to his people while they were idolaters. Then in some detachment or battle of the Muslims he would be killed by mistake (at the hands of the Muslims), so it was obligatory upon (the one who killed him) to free a believing slave. And (likewise): 'And if he was from a people between whom and you there is a treaty, then blood money is to be handed over to his family and a believing slave is to be freed' [Surah an-Nisa: 92]." He said: "(This means that) the man himself is under a treaty and his people are also people of a treaty, so his blood money is handed over to them, and the one who killed him frees a slave."
This hadith has an authentic (sahih) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.