Narrated Abdullah Ibn Abbas:
In Surat al-Muzzammil (73), the verse: "Keep vigil at night but a little, a half thereof" (2-3) has been abrogated by the following verse: "He knoweth that ye count it not, and turneth unto you in mercy. Recite then of the Qur'an that which is easy for you" (v.20). The phrase "the vigil of the night" (nashi'at al-layl) means the early hours of the night. They (the companions) would pray (the tahajjud prayer) in the early hours of the night.
He (Ibn Abbas) says: It is advisable to offer the prayer at night (tahajjud), prescribed by Allah for you (in the early hours of the night). This is because when a person sleeps, he does not know when he will awake. The words "speech more certain" (aqwamu qilan) means that this time is more suitable for the understanding of the Qur'an. He says: The verse: "Lo, thou hast by day a chain of business" (v.7) means engagement for long periods (in the day's work).