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, you have by day a chain of business" (5:7) means engagement for long periods (in the day's work).
In-book : Book 5, Hadith 55
English translation : Book 5, Hadith 1299