Mukhtasar Muslim (Abridged) 1200 — Ruling if two caliphs are pledged

Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: When allegiance is pledged to two caliphs, kill the one to whom allegiance was pledged later (because his caliphate is invalid while the first caliph exists).