Is a person who does not pray Qiyam al-Layl (i.e., does not pray Witr) considered a fasiq (transgressor), according to the saying of Imam Ahmad and other scholars, and is this saying the most preponderant (rajih)?

The Witr prayer is a confirmed Sunnah (definition) (Prophetic tradition), not an obligation, according to the majority of scholars. One who occasionally misses it is not considered a fasiq (transgressor). However, regarding one who abandons it entirely, some scholars have held that such a person is a fasiq and their testimony should be rejected. This is because by consistently abandoning the Sunnah, they are showing disinterest in it, and the Prophet, peace and blessings be upon him, said: "Whoever turns away from my Sunnah is not of me." Similarly, one who consistently abandons Sunnah prayers and regular supererogatory prayers, like the two rak'ahs before Fajr, will have their testimony rejected due to their negligence in religious matters and their lack of concern for important duties.