If anyone is afraid that he may not get up in the latter part of the night, he should observe Witr in the first part of it; and if anyone is eager to get up in the last part of it, he should observe Witr at the end of the night, for prayer at the end of the night is witnessed (by the angels) and that is preferable.  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ خَافَ أَنْ لاَ يَقُومَ مِنْ آخِرِ اللَّيْلِ فَلْيُوتِرْ أَوَّلَهُ وَمَنْ طَمِعَ أَنْ يَقُومَ آخِرَهُ فَلْيُوتِرْ آخِرَ اللَّيْلِ فَإِنَّ صَلاَةَ آخِرِ اللَّيْلِ مَشْهُودَةٌ وَذَلِكَ أَفْضَلُ وَقَالَ أَبُو مُعَاوِيَةَ مَحْضُورَةٌ  

[Machine] The Messenger of Allah ﷺ said, "Whoever thinks that he will not wake up for the later part of the night, let him pray the first part of it. And whoever thinks that he will wake up for the later part of the night, let him pray the later part. Verily, the prayer of the later part of the night is prohibited (makruh), and it is the best."  

قَالَ رَسُولُ اللهِ ﷺ مَنْ ظَنَّ مِنْكُمْ أَنْ لَا يَسْتَيْقِظَ آخِرَهُ فَلْيُوتِرْ أَوَّلَهُ وَمَنْ ظَنَّ مِنْكُمْ أَنَّهُ يَسْتَيْقِظُ آخِرَهُ فَلْيُوتِرْ آخِرَهُ فَإِنَّ صَلَاةَ آخِرِ اللَّيْلِ مَحْضُورَةٌ وَهِيَ أَفْضَلُ  

"مَنْ خَافَ أَنْ لا يَقُومَ مِنْ آخِرِ اللَّيلِ فَليُوتِرْ أَوَّلهُ، وَمَنْ طَمِعَ أَنْ يَقُومَ آخِرَهُ فَليُوتِرْ آخِرَ اللَّيلِ، فَإِنَّ صَلاةَ آخِرِ اللَّيلِ مَشْهُودَةٌ، وَذَلِكَ أَفْضَلُ".