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  

He who amongst you is afraid that he may not be able to get up at the end of the night should observe Witr (in the first part) and then sleep, and he who is confident of getting up and praying at night (i. e. Tahajjud prayer) should observe it at the end of it, for the recitation at the end of the night is witnessed*, and that is better. *: meaning, "by angels" (Sharh an-Nawawi)  

سَمِعْتُ النَّبِيَّ ﷺ يَقُولُ أَيُّكُمْ خَافَ أَنْ لاَ يَقُومَ مِنْ آخِرِ اللَّيْلِ فَلْيُوتِرْ ثُمَّ لْيَرْقُدْ وَمَنْ وَثِقَ بِقِيَامٍ مِنَ اللَّيْلِ فَلْيُوتِرْ مِنْ آخِرِهِ فَإِنَّ قِرَاءَةَ آخِرِ اللَّيْلِ مَحْضُورَةٌ وَذَلِكَ أَفْضَلُ  

“Whoever among you fears that he will not wake up at the end of the night, let him pray Witr at the beginning of the night, then go to sleep. Whoever hopes that he will wake up at the end of the night, let him pray Witr at the end of the night, for recitation (of the Qur’an) at the end of the night is attended (by the angels), and that is better.” (Using translation from Ibn Mājah 1187)   

قَالَ رَسُولُ اللهِ ﷺ مَنْ خَشِيَ مِنْكُمْ أَنْ لَا يَقُومَ مِنْ آخِرِ اللَّيْلِ فَلْيُوتِرْ مِنْ أَوَّلِ اللَّيْلِ ثُمَّ ليَِرْقُدْ وَمَنْ طَمِعَ مِنْكُمْ فِي أَنْ يَقُومَ مِنْ آخِرِ اللَّيْلِ فَلْيُوتِرْ مِنْ آخِرِ اللَّيْلِ فَإِنَّ قِرَاءَةَ آخِرِ اللَّيْلِ مَحْضُورَةٌ وَذَلِكَ أَفْضَلُ  

“Whoever among you fears that he will not wake up at the end of the night, let him pray Witr at the beginning of the night, then go to sleep. Whoever hopes that he will wake up at the end of the night, let him pray Witr at the end of the night, for recitation (of the Qur’an) at the end of the night is attended (by the angels), and that is better.” (Using translation from Ibn Mājah 1187)   

عَنْ رَسُولِ اللَّهِ ﷺ قَالَ «مَنْ خَشِيَ مِنْكُمْ أَنْ لَا يَقُومَ مِنْ آخِرِ اللَّيْلِ فَلْيُوتِرْ مِنْ أَوَّلِ اللَّيْلِ وَمَنْ طَمِعَ مِنْكُمْ أَنْ يَقُومَ آخِرَ اللَّيْلِ فَلْيُوتِرْ آخِرَ اللَّيْلِ فَإِنَّ قِرَاءَةَ آخِرِ اللَّيْلِ مَحْضُورَةٌ وَذَلِكَ أَفْضَلُ»  

“Whoever among you fears that he will not wake up at the end of the night, let him pray Witr at the beginning of the night, then go to sleep. Whoever hopes that he will wake up at the end of the night, let him pray Witr at the end of the night, for recitation (of the Qur’an) at the end of the night is attended (by the angels), and that is better.” (Using translation from Ibn Mājah 1187)   

قَالَ رَسُولُ اللهِ ﷺ مَنْ خَافَ أَنْ لَا يَسْتَيْقِظَ آخِرَ اللَّيْلِ فَلْيُوتِرْ أَوَّلَهُ ثُمَّ لِيَرْقُدْ وَمَنْ طَمِعَ أَنْ يَسْتَيْقِظَ مِنْ آخِرِ اللَّيْلِ فَلْيُوتِرْ مِنْ آخِرِ اللَّيْلِ فَإِنَّ قِرَاءَةَ آخِرِ اللَّيْلِ مَحْضُورَةٌ وَذَلِكَ أَفْضَلُ  

"مَنْ خَشِيَ مِنْكُمْ أَن لا يَقُومَ مِنْ آخِر اللَّيلِ [فَليُوتِر مِنْ أَوَّلِ اللَّيل] ، وَمَنْ طَمِعَ مِنْكُم أَنْ يَقُومَ آخِرَ اللَّيلِ، فَليُوتِر آخِرَ اللَّيل، فَإِنَّ قِرَاءَةَ آخِر اللَّيلِ مَحْضُورَةٌ، وَذَلِكَ أَفْضَلُ".  

"مَنْ ظَنَّ مِنْكُمْ أَنْ لا يَسْتَيقِظَ آخِرَ اللَّيلِ، فَلْيُوتِرْ أَوَّلَهُ، وَمَنْ ظَنَّ أَنَّهُ يَسْتَيقِظُ آخِرَهُ، فَلْيُوتِرْ آخِرَهُ، فَإِنَّ صَلاةَ آخِر اللَّيلِ مَحْضُورَةٌ وَهي أَفْضَلُ".