بُلُوغُ الْمَرَامِ
Bulugh al-Maram 390
The Book of Prayer
وَعَنْ جَابِرٍ قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ مَنْ خَافَ أَنْ لَا يَقُومَ مِنْ آخِرِ اَللَّيْلِ فَلْيُوتِرْ أَوَّلَهُ, وَمَنْ طَمِعَ أَنْ يَقُومَ آخِرَهُ فَلْيُوتِرْ آخِرَ اَللَّيْلِ, فَإِنَّ صَلَاةَ آخِرِ اَللَّيْلِ مَشْهُودَةٌ, وَذَلِكَ أَفْضَلُ } رَوَاهُ مُسْلِمٌ . [1] .
Narrated Jabir (RA): Allah's Messenger (ﷺ) said: "If anyone is afraid that he may not get up in the latter part of the night, he should offer Witr in the first part of it; and if anyone expects to get up in the last part of it, he should offer Witr at the end of the night, for prayer at the end of the night is witnessed (by the angels) and that is preferable." [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 294