There is no one worthy of worship but God Almighty and Muhammad is his servant and messenger
“Submission to God Almighty” is the exact meaning and definition of “Islam”. One who practices this is known as a “Muslim” which means “anyone who submits himself to God Almighty”. Islam is the continuation of the faith of all prophets, who because of their submission to God Almighty were Muslims, having brought the message of peace and submission to different peoples at different times. Such peace and submission are not attained except through worship of God alone, and because of this He sent down revelation to message to Prophets and Messengers to convey his message to mankind. The line of prophets starts with Adam, down through Abraham and Isaac, Jacob and Joseph, John and Jesus, and ending with the final messenger Muhammad.
Islam is, because of this, considered to be one of the three Abrahamic faiths (i.e. those that recognize Abraham as a religious father-figure and recognize his manner of worship, worshiping God Almighty alone without intercessor or partner). Muhammad being the last of these Prophets and Messengers, faith in his prophecy is integral to faith in God, as he was commissioned to relay that message to mankind.