How can one become among the truthful (Siddiqin) and steadfast in night prayer (Qiyam al-Layl), and what is its reward?

A Siddiq (the Truthful One) is he who is truthful in his words and deeds, and who confirms the truth with his words and actions. Truthfulness in speech means the alignment of the tongue with one's words. In actions, it means the alignment of deeds with commands and adherence (to divine guidance). In states, it means the alignment of the heart's actions and the limbs with sincerity and the exertion of utmost effort. The complete faith of a Siddiq is perfected through knowledge, action, calling to Allah, and patience. Therefore, whoever wishes to attain the rank of Siddiqiyyah (Truthfulness) should strive to be truthful in words and actions and to achieve complete faith in Allah and the Last Day.