2278 is very clear that Abraham was the first Muslim.
2278
You shall strive for the cause of GOD as you should strive for His cause. He has chosen you and has placed no hardship on you in practicing your religion - the religion of your father Abraham. He is the one who named you "Submitters" (Muslims) originally. Thus, the messenger shall serve as a witness among you, and you shall serve as witnesses among the people. Therefore, you shall observe the Contact Prayers (Salat) and give the obligatory charity (Zakat), and hold fast to GOD; He is your Lord, the best Lord and the best Supporter.
Wa Jhid F Allhi aqqa Jihdihi Huwa Ajtabkum Wa M Jaala Alaykum F Ad-Dni Min arajin Millata 'Abkum 'Ibrhma Huwa Sammkum Al-Muslimyna Min Qablu Wa F Hdh Liyakna Ar-Raslu Shahdan Alaykum Wa Takn Shuhad'a Ala? An-Nsi Fa'aqm A-alata Wa 't Az-Zakata Wa Ataim Billhi Huwa Mawlkum Fanima Al-Mawla? Wa Ni`ma An-Naru
This religion is also called Millath-e-Ibrahim which means Religion of our Father Abraham.
Adam was the first submitter of the spiritual religion of submitting to God.
Abraham was the first submitter of the physical religion Islam.
Salaam