Abu Burdah (may Allah have mercy on him) said, regarding the story in the previous hadith: The man was brought to Sayyiduna Abu Musa (may Allah be pleased with him), and he invited him (to Islam) for about twenty nights. Then Sayyiduna Mu'adh (may Allah be pleased with him) came and he too invited him to Islam, but he refused, so he killed him.
The Shaykh said: It has been narrated to us that Sayyiduna Abu Bakr (may Allah be pleased with him) told Sayyiduna Khalid bin al-Walid (may Allah be pleased with him): First present Islam to them; if they accept, well and good, otherwise kill them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16883