Lexicology and Quranic usage
The plural form of the word "siddiq" is "siddiqun". "Siddiq" in terms of the Arabic grammar is an exaggerative form from the root "s-d-q" (ص-د-ق), meaning a person who always exhibits "sidq" (Arabic: الصدق) that is, truth or honesty.
In the Quran, the single form of "siddiq" has been used four times and its plural form was used two times. Ibrahim (a) and Idris (a) have been characterized in the Quran as "siddiq", and Maryam (a) (St Mary the Virgin) has been characterized as "siddiqa" (the female form of "siddiq"). In hadiths, Imam 'Ali (a) has been characterized as "Siddiq" and Fatima al-Zahra (a) as "Siddiqa" or "al-Siddiqa al-Kubra".
Position of Siddiqun
According to Raghib al-Isfahani, siddiqun are people who are very close to prophets with respect to virtues.
According to a hadith in Shiite sources, when a "siddiqa" (a female siddiq) dies, no one can do her washing rituals except a siddiq. For example, Fatima al-Zahra (a)'s washing rituals were done by Imam 'Ali (a), and Maryam (a)'s washing rituals were done by Jesus (Isa) (a).
In Shiite and Sunni sources, Imam 'Ali (a) has been characterized by the Prophet (s) as al-Siddiq al-Akbar. The Prophet (s) mentioned 'Ali b. Abi Talib (a), Mu'min Al Fir'awn (the believer from the Pharaoh household) and Habib al-Najjar as Siddiq, emphasizing that 'Ali (a) is the greatest ("Akbar") among them.
According to Sunni sources, Abu Bakr is also characterized as "siddiq" in Hadith al-Mi'raj or, on other accounts, he was called so since the period of Jahiliyya (ignorance). However, the hadith has been rejected as unreliable by some Sunni scholars. Ibn Jawzi cited the hadith as a fake one in his book, al-Mawdu'at.
Shiite scholars reject the title of "siddiq" for Abu Bakr. Moreover, they appeal to hadiths from Sunni sources to show that this title as well as the title, Faruq, are exclusively those of 'Ali b. Abi Talib (a), since during his caliphate, he stated on the minbar of Basra that the titles are exclusive his.
The material for writing this article is mainly taken from صدیق in Farsi wikishia.