Shi'a in the Quran

The word "Shi'ah" means "followers; members of a party". Allah has mentioned in the Qur'an that some of His righteous servants were Shi'ah of His other righteous servants:

And most surely Abraham was among the Shi'ah of him. (Qur'an 37:83)

And he (Moses) went into the city at a time when people (of the city) were not watching, so he found therein two men fighting, one being of his Shi'ah and the other being his enemy, and the one who was of his Shi'ah cried out to him for help against the one who was of his enemy. (Qur'an 28:15)

Thus Shi'ah is an official word used by Allah in His Qur'an for His high rank Prophets as well as their followers.

If one is a Shi'ah (follower) of the most righteous servants, then there is nothing wrong with being Shi'ah. On the other hand, if one becomes the Shi'ah of a tyrant or a wrong-doer, he shall meet with the fate of his leader. The Qur'an indicates that on the Day of Judgment people will come in groups, and each group would have its leader (Imam) in front of it. Allah says:

(Remember) the day when we will call every people with their Imam. (Qur'an 17:71)

On the Day of Judgment, the destiny of the "followers" of each group depends on the destiny of their Imam (provided that they really followed that Imam). Allah mentioned in the Qur'an that there are two types of Imams:

And We made them Imams who call to the fire, and on the Day of Resurrection they shall not be assisted. And We caused a curse to follow them in this world, and on the Day of Resurrection they shall be of those made to appear hideous. (Qur'an 28:41-42)

The Qur'an also reminds that there are Imams who are appointed by Allah as Guides for the mankind:

And We made of them Imams to guide by Our command when they were patient, and they were certain of Our communications. (Qur'an 32:24)

Certainly, the true followers (Shi'ah) of these Imams will be the real prosperous people on the Day of Resurrection.

Shi'ah in the Hadith

In the history of Islam, "Shi'ah" has been especially used for the followers of Imam Ali (a). This phrase is not something invented later! The first individual who used this term was the Messenger of Allah himself. When the following verse of the Qur'an was revealed:

(As for) those who believe and do good, surely they are the best of creatures. (Qur'an 98:7)

The Prophet (s) said to Ali: "It is for you and your Shi'ah."

He further said: "I swear by the one who controls my life that this man (Ali) and his Shi'ah shall secure deliverance on the Day of Resurrection."

Notes:

Jalal al-Din al-Suyuti, Tafsir al-Durr al-Manthur, (Cairo) vol. 6, p. 379.
Ibn Jarir al-Tabari, Tafsir Jami' al-Bayan, (Cairo) vol. 33, p. 146.
Ibn Asakir, Ta'rikh Dimashq, vol. 42, p. 333, p. 371.
Ibn Hajar al-Haythami, al-Sawa'iq al-Muhriqah, (Cairo) Ch. 11, section 1, pp. 246-247.
The Prophet (s) said: "O Ali! (On the day of Judgment) you and your Shi'ah will come toward Allah well-pleased and well-pleasing, and there will cometo Him your enemies angry and stiff-necked (i.e., their head forced up).
Ibn al-'Athir, al-Nihaya fi gharib al-hadith, (Beirut, 1399), vol. 4 p. 106.
al-Tabarani, Mu'jam al-Kabir, vol 1 p 319.
al-Haythami, Majma' al-Zawa'id, vol. 9, number 14168
The Prophet (s) said: "Glad tiding O Ali! Verily you and your Shi'ah will be in Paradise."
Ahmad Ibn Hanbal, Fadha'il al-Sahaba, (Beirut) vol. 2, p. 655-
Abu Nu'aym al-Isbahani, Hilyatul Awliya, vol. 4, p. 329.
al-Khatib al-Baghdadi , Tarikh Baghdad, (Beirut) vol. 12, p. 289.
al-Tabarani, Mu'jam al-Kabir, vol. 1, p. 319.
al-Haythami, Majma' al-Zawa'id, vol. 10, pp. 21-22.
Ibn 'Asakir, Ta'rikh Dimashq, vol. 42, pp. 331-332.
Ibn Hajar al-Haythami, al-Sawa'iq al-Muhriqah, (Cairo) Ch. 11, section 1, p. 247

References:
By: Tahir & thenlightenment