Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) seized some people in Kufah who were propagating the religion of Musaylimah the Liar. He asked Sayyiduna Uthman (may Allah be pleased with him) about them, and the reply came: Present Islam to them and invite them to Tawhid (the Oneness of Allah); if they accept, release them, and whoever does not accept, kill him. So Sayyiduna Ibn Mas'ud (may Allah be pleased with him) did exactly that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16852