Passage: The prolonged and escalating Byzantine–Sassanid wars of the 6th and 7th centuries and the recurring outbreaks of bubonic plague (Plague of Justinian) left both empires exhausted and vulnerable in the face of the sudden emergence and expansion of the Arabs. The last of the wars between the Roman and Persian empires ended with victory for the Byzantines: Emperor Heraclius regained all lost territories, and restored the True Cross to Jerusalem in 629.Nevertheless, neither empire was given any chance to recover, as within a few years they found themselves in conflict with the Arabs (newly united by the Islam), which, according to Howard-Johnston, "can only be likened to a human tsunami". According to George Liska, the "unnecessarily prolonged Byzantine–Persian conflict opened the way for Islam".In late 620s, the Islamic Prophet Muhammad had already managed to unify much of Arabia under Muslim rule via conquest as well as making alliances with neighboring tribes, and it was under his leadership that the first Muslim-Byzantine skirmishes took place. Just a few months after Emperor Heraclius and the Persian general Shahrbaraz agreed on terms for the withdrawal of Persian troops from occupied Byzantine eastern provinces in 629, Arab and Byzantine troops confronted each other at the Mu'tah in response to the murder of Muhammad's ambassador at the hands of the Ghassanids, a Byzantine vassal kingdom. Muhammad died in 632 and was succeeded by Abu Bakr, the first Caliph with undisputed control of the entire Arabian Peninsula after the successful Ridda Wars, which resulted in the consolidation of a powerful Muslim state throughout the peninsula.

Question: Who succeeded Muhammad?
Abu Bakr