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

Yes, all the Prophets preached monotheism. The message with which all the Prophets were sent was that of pure monotheism, emphasizing the belief in one God. Throughout history, every Prophet has conveyed this fundamental truth, calling their communities toward the worship of the one true God. The mission of all the prophets right from Adam to Christ was one and the same—of establishing the ideology of monotheism in the world, so that man may worship one God alone. As we know, there came a large number of prophets in ancient times, but the message of monotheism remained the main doctrine of each Messenger. Despite the challenges they faced, their mission consistently focused on establishing monotheism as the core of their teachings.

Source: Islam Rediscovered

A believing Muslim is bound to respect all the prophets. He cannot differentiate between the messengers of God. This is an article of the Islamic faith.

God had sent a prophet or messenger to every group and community of people in this world. All the messengers of God enjoy an equal status. In the chapter al-Baqarah (The Heifer), the Quran says:

We do not differentiate between any of His messengers. We hear and obey. Grant us Your forgiveness, Lord, to You we shall all return! (Quran 2:285)

The Prophet is recorded to have said, Do not give me superiority over other prophets. (Sahih al-Bukhari, hadith no. 4638)

This Quranic verse and the hadith relate directly to the messengers or the prophets of God. But both also have a broader application, according to which, every person is worthy of respect, regardless of creed or culture. Such belief promotes a culture of equal respect among mankind. Difference in belief is necessarily subject to discussion, but not a reason for discrimination. If your belief system is different from that of others, you have every right to engage in discussion and dialogue on the subject, but in the process, you have no right to show disrespect to others. This holds true even if you believe that your ideology enshrines the supreme truth. The culture of discrimination is, after all, quite alien to a divine religion.

Source: The Seeker’s Guide

A believing Muslim is bound to respect all the prophets. He cannot differentiate between the messengers of God. God had sent a prophet or messenger to every group and community of people in this world. All the messengers of God enjoy an equal status. In the chapter al-Baqarah (The Heifer), the Quran says:

“We do not differentiate between any of His messengers. We hear and obey. Grant us Your forgiveness, Lord, to You we shall all return!” (2:285)

The Prophet is recorded to have said, “Do not give me superiority over other prophets.” (Sahih al-Bukhari, hadith no. 4638)

This Quranic verse and the hadith relate directly to the messengers or the prophets of God. But both also have a broader application, according to which, every person is worthy of respect, regardless of creed or culture. Such belief promotes a culture of equal respect among mankind. Difference in belief is necessarily a subject to discussion, but not a reason for discrimination. If your belief system is different from that of others, you have every right to engage in discussion and dialogue on the subject, but in the process, you have no right to show disrespect to others. This holds true even if you believe that your ideology enshrines the supreme truth. The culture of discrimination is, after all, quite alien to a divine religion.

Source: The Seeker’s Guide