Musannaf Ibn Abi Shaybah 34932 — What they said on a man who accepts

Abu Alaqah (may Allah have mercy on him) said that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) sent out a detachment. They found a man from among the Muslims who had become a Christian after accepting Islam, and they killed him. Sayyiduna Umar (may Allah be pleased with him) was informed of this, and he (may Allah be pleased with him) asked: "Did you invite him to Islam?" They said: "No." He (may Allah be pleased with him) said: "Then I indeed declare myself free before Allah of his blood."