What is meant by "prayer" in the Prophet's (PBUH) saying, "If he prays, a knot is untied," in the aforementioned Hadith; is it the prayer of Qiyam (night vigil) or the Fajr prayer?

The agreed-upon Hadith narrated by Abu Hurairah, which was referred to, intends "Salat" (prayer) therein to mean the night vigil (Qiyam al-Layl). This is indicated by the Prophet's (peace be upon him) saying: "If a long night overtakes you, then sleep, and when you awaken," and his saying at the end of the Hadith: "he will rise with a good soul, otherwise he will rise with a wicked soul." Al-Bukhari and others have titled a chapter concerning this Hadith as "The knot tied by Satan on the nape of a servant's neck if he does not pray at night," and it contains an encouragement for Qiyam al-Layl, indicating that the supererogatory night prayer is legislated and desirable.