Sunnat
This topic has been discussed at length in the following video: https://www.youtube.com/watch?v=ilGNUdC0pCs
It’s a misconception that there were no Muslims. Islam started with prophet Adam AS, then through Ismael AS Arabs received the sunnah of prophet Ibrahim AS. Even before prophethood, prophet Muhammad SAW was not from the polytheists and followed the Abrahamic tradition also known as the deen-e-hanifah. It is true that they had corrupted the deen with lots of innovations, but prophet Muhammad SAW cleared those out. In the Quran God specifically asks prophet Muhammad SAW and his followers to follow the creed of prophet Ibrahim. Following are some of the relevant ayahs from the Quran that might help: Quran 16:123 Translation by Asad And lastly, We have inspired thee, [O Muhammad, with this message:] “Follow the creed of Abraham, who turned away from all that is false, and was not of those who ascribe divinity to aught beside God; Quran 3:68 Translation by Asad Abraham was neither a “Jew” nor a “Christian”, but was one who turned away from all that is false, having surrendered himself unto God; and he was not of those who ascribe divinity to aught beside Him. Quran 22:78 Translation by Asad And strive hard in God’s cause with all the striving that is due to Him: it is He who has elected you [to carry His message], and has laid no hardship on you in [anything that pertains to religion, [and made you follow] the creed of your forefather Abraham. It is He who has named you in bygone times as well as in this [divine writ] – “those who have surrendered themselves to God”, so that the Apostle might bear witness to the truth before you, and that you might bear witness to it before all mankind. Thus, be constant in prayer, and render the purifying dues, and hold fast unto God. He is your Lord Supreme: and how excellent is this Lord Supreme, and how excellent this Giver of Succour!