Yes, all the Prophets were Muslims in the sense that they submitted to the will of God and preached the message of monotheism. According to Islamic belief, every Prophet conveyed the same fundamental truth—that there is only one God and that all human beings are accountable to Him for their actions. Every one of them was an upright man guided by God along a straight path and as such was superior to all others. Therefore, while they may have had different laws and practices specific to their communities, their core message of submission to God aligns them all with Islam. Chapter 6 of the Quran mentions a number of these prophets by name. They are Ibrahim (Abraham), Ishaaq (Issac), Yaqub (Jacob), Nuh (Noah), Dawood (David), Sulaiman (Solomon), Ayyub (Job), Yusuf (Joseph), Musa (Moses), Haroon (Aaron), Zakariya, Isa (Jesus), Ismail (Ishmael), Yahya (John the Baptist), Yunus (Jonah), and Lut (Lot).   God also instructed us not to differentiate between the prophets as they essentially brought the same message to mankind. (Quran, 2:285). Prophet Muhammad was instructed by God to follow the guidance of the Prophets before him. By extension, all the followers of Prophet Muhammad must also seek guidance from the lives of the other prophets. By this, it is meant that each one of us must evaluate the situation he faces and then whilst examining the lives of other prophets, determine in which prophet he can find a similar example. His response to the situation should be the same as that of the prophets in whose example he found the similarity.

Source: The Spirit of Islam Magazine