No man started Islam it was ALLAH Subhana Wa Ta'ala The Creator Of The Heavens and The Earth.
No human is responsible of founding, creating, or establishing Islam religion. Islam religion is God religion. Islam religion is per God (Allah) revelation of Quran to prophet Muhammad through the Angel Gabriel. Quran revelation started in year 610 AD and continued until year 632 AD (year of death of the prophet).
Islam religion, as defined above, should not be confused by Islam, as an Arabic word, that in its universal sense means peace and full submission to God (Allah), the one and only one God with no partner, no father, no son, no companion, and no resemblance. Islam in this sense is the main God mission to all prophets since start of mankind; including Noah, Abraham, Moses, Jesus, and Muhammad. However, Judaism and Christianity are the God religions per God revelation of , respectively, Torah and Bible to Moses and Jesus as well as the very name Islam is God religion per God revelation of Quran to prophet Muhammad.
Refer to related questions on why Quran was revealed and the golden rules in Quran.