Ibn Umar (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) killed a Muslim in return for a dhimmi and said: "I am the one most faithful in fulfilling my covenant of protection."
There are two defects in this narration: (1) Making it connected through Ibn Umar (may Allah be pleased with them both) is a mistake; rather, it is a mursal narration of (Ibn) al-Baylamani from the Prophet (peace and blessings be upon him). (2) Ibrahim narrates it from Ibn al-Munkadir, and the fault in it is attributed to Ammar bin Matar al-Rahawi, who used to switch chains of narration and steal hadith; he is a narrator who has fallen below the level of being used as evidence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15917