Ahl al-Bayt (a) (Arabic: أهل البیت) is a special title for some members of the family of the noble Prophet (s) who are referred to in the al-Tathir and al-Mubahala verses and include Imam 'Ali (a), Lady Fatima (s), Imam al-Hasan (a) and Imam al-Husayn (a) and the nine infallible Imams (a) who were the children of Imam al-Husayn (a).
From the Shi'a viewpoint, Ahl al-Bayt (a) are infallible and superior to all other companions of the Prophet (s) in regards to taqwa (God-wariness) and divine supports and loving them is mandatory upon all Muslims. According to Shi'a teachings, Ahl al-Bayt (a) have guardianship and leadership upon Muslims and Muslims must regard them as their own authorities and refer to them in their religious issues.
Superiority of the Ahl al-Bayt (a) over others can easily be understood from the hadith al-Thaqalayn, because the Prophet (s) put them beside the Qur'an, called the Qur'an as "al-Thiql al-Akbar" and Ahl al-Bayt (a) as "al-Thiql al-Asghar" and never put another person beside the Qur'an. Therefore, the Ahl al-Bayt (a) are superior to others the same way as the noble Qur'an is superior over other books.
The al-Mubahala verse implies the superiority of Ahl al-Kisa' over other companions of the Prophet (s) as well; since, according to this verse, God has given the Prophet (s) mission to choose some people from among children, Muslim men and women to accompany him in Mubahala with the Christians of Najran; and the Prophet (s) chose Imam 'Ali (a), Lady Fatima (a) and al-Hasan (a) and al-Husayn (a).
There is no doubt that those are chosen for Mubahala have great position in faith and closeness to God.
Abu Riyah, the servant of Umm Salama, narrated from the Prophet (s): "If there were other people more honorable than 'Ali (a), Fatima (s), al-Hasan (a) and al-Husayn (a), God would have ordered me to do mubahala with their help; and they are the best of people."