Ṣidq (truthfulness). In all these matters, the one characterized by ṣidq is called a ṣiddīq (one of utmost truthfulness), because he has reached a very advanced station in the matter of ṣidq.
1. Ṣidq al-lisān (truthfulness of the tongue) occurs in reports or in the content of reports. A report is either related to the past or to the future. Whether one keeps one's promise is related to reports about the future. Every servant must keep his word. A person must always speak the truth. This is the clearest and most well-known form of truthfulness. One who guards his tongue from reporting contrary to the reality of things is called ṣādiq (truthful). (Iḥyāʾ, 117)
Ṣidq al-ʿazm (truthfulness in resolve) is as follows: Resolve sometimes precedes action. For example, a person may say to himself, "If Allah grants me wealth, I will give all or part of it in charity; if I encounter the enemy, I will fight with all my strength and not care about death; if Allah grants me a position, I will not rebel against Him by inclining toward oppression and people, but will judge with absolute justice." Such resolve sometimes arises from within, and this is a firm and truthful resolve (in which there is no doubt or hesitation).
Sometimes, however, there is a trace of hesitation and weakness in this resolve, and this state is contrary to truthfulness. Regarding this truthfulness, it is said just as one says, "So-and-so has a ṣahwatun ṣādiqatin (a strong and complete appetite or desire)," where completeness (being free from deficiency) consists in strength. Similarly, if it does not come from a strong foundation, the appetite of a sick person is called false. (Iḥyāʾ, 119)
Ṣidq in the maqāmāt al-dīn (stations of religion) is truthfulness in all matters such as fear, hope, reverence, asceticism, riḍā (contentment with God's decree), tawakkul (reliance upon God), maḥabba (love), and the like. Each of these has a beginning, and by its manifestation, it acquires this name. Then there is its ultimate goal and ḥaqīqa (inner reality). The muḥaqqiq (one of realization) who is truthful attains to its inner reality. When something becomes established and its reality is completed, its possessor is called a true (real) ṣādiq. (Iḥyāʾ, IV, 121; Qudāma, 395) Just as one says, "So-and-so is truthful in the matter of fighting." Thus, it is said, "The fear of the ṣādiq is true fear." (Iḥyāʾ, 122)
Ṣidq is the congruence of the hidden and the manifest (the inner and the outer)... (Adāb2, 20)
Jaʿfar Khuldī said: I asked Abū'l-Qāsim Junayd, "Is there a difference between ikhlas (sincerity) and ṣidq?" He replied, "Yes, ṣidq is the root and the first. Ikhlas is the branch and is subordinate to ṣidq. The second is always subordinate to the first." He then continued: "There is a difference between them, because ikhlas only arises after one begins to act. Ikhlas is to make one's sincerity pure, to be pure (in intention) in this matter. The ikhlas described by the Malāmatiyya is the striving to purify ikhlas, which is the state of the Sufis. To possess (be a sahib of) pure ikhlas in the matter of purifying ikhlas is only possible when one's ikhlas is pure."