1. Shia means the followers. In Quran Allah called the sincere followers of the Prophet Musa as his Shia (The person who was from his Shia seeked help from him against the person who was from his enemies) (فاستغاثه الذي هو من شيعته على الذي من عدوه) (Sura Al-Qasas, Verse 15.)
2. Allah in Quran called prophet Ebrahim as a Shia (sincere follower) of the prophet Noah. ( وان من شيعته لابراهيم )(And verily, among his Shia (followers) are Ebrahim). (Sura al-Saffaat, verse 83.)
3. The Prophet Muhammad (SAWA) called the sincere obedient Muslims as Shia of Ali and said that they will be the winners in the Day of Judgment.
( Tafseer al-Tabari, Vol 30, Page 146)
(Tafseer al-Dorr al-Manthoor by al-Soyooti 6/379) and many other Sunni books.
4. Shia Muslims are the sincere followers of the Prophet Muhammad (SAWA) who took his Sunnah and Tafseer of Quran from his holy Progeny Ahlul Bayt (AS) being he most authentic source.