According to Wikipedia, Adam is considered the first Muslim by Muslims. This is because Adam is called a prophet of God and the Quran also says that all the prophets are of the same faith (Islam). It is Abraham who is considered to have built the first house of worship on Earth. All that being said, there are multiple verses in the Quran that suggest that Muhammad was truly the first Muslim, such as surah 6:163 and 39:11-12.
So neither Wikipedia nor the Quran seem to suggest that Abraham was considered the first Muslim, though he is highly regarding as a prophet and is important in the text.