Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) states: If two men enter Islam and then cut themselves off from one another, one of them will be leaving Islam until he returns. (The narrator says:) That is, the wrongdoer will be like this.
Al-Bazzar recorded it; its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 12970