Sayyiduna Abdullah bin Amr (may Allah be pleased with him) reports this as a marfu' hadith: (The Prophet (peace and blessings be upon him) said:) "Reviling a Muslim is like one who stands at the brink of destruction."
Al-Bazzar recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 13011