Al-Sādiqīn Verse (Arabic: آیة الصادقين, verse of the truthful), is the title of verse 119 of sura al-Tawba which commands believers to be with "al-sadiqin" (the truthful) and follow them. Al-Zamakhshari, one of Sunni exegetes, says: "the truthful are who are truthful in their beliefs, word and behavior". (Al-Kashshaf, vol.2 p.220) According to the belief of Shi'a, the truthful refers to Ahl al-Bayt (a). This verse is one of "Qur'anic" reasons of imamate of the twelve Imams (a).
Truth means the quality of being based on fact. (Al-Jurjani, Al-Ta'airf, p.95) In other words, truth is a description for a word which is based on fact. Belief, and promise are also called "word", so this matters also could be described as truth. When someone has beliefs according to the fact, or acts as he promises, he is in his beliefs and promises truthful. (Tabataba'i, Al-Mizan, vol.9 p.402)