Before the Shahada

One becomes a Muslim by reciting the canned prayer (the shahada) that there is one God and Muhammad is his messenger. Yet Muslims claim Abraham, Moses and Jesus were Muslims. How did they claim Muhammad was God's messenger when he had not yet been born?  There is no record of a shahada of Abraham, or a shahada of Moses.  Jesus had no shahada, nor did he pray to a stone in Mecca.  Muhammad's shahada sounds like poppycock to me.