Abd al-Rahman bin al-Baylamani (may Allah have mercy on him) narrates, attributing it to the Prophet (marfu'), that a Muslim killed a dhimmi, and the Prophet (peace and blessings be upon him) said: "I am the one most faithful in fulfilling a covenant of protection." Then he (peace and blessings be upon him) had that Muslim killed. This narration is munqati' (has a broken chain), and its narrator is not trustworthy.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15918