The (Voluntary) Night Prayer Is Performed In Units Of Two

Narrated 'Abdullah bin 'Umar:
A man asked the Messenger of Allah ﷺ about the prayer at night. The Messenger of Allah ﷺ said: Prayer during the night should consist of pairs of rak'ahs, but if one of you fears the morning is near he should pray one rak'ah which will make his prayer an odd number for him.

حَدَّثَنَا الْقَعْنَبِيُّ، عَنْ مَالِكٍ، عَنْ نَافِعٍ، وَعَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، : أَنَّ رَجُلاً، سَأَلَ رَسُولَ اللَّهِ ﷺ عَنْ صَلاَةِ اللَّيْلِ، فَقَالَ رَسُولُ اللَّهِ ﷺ : " صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى، فَإِذَا خَشِيَ أَحَدُكُمُ الصُّبْحَ صَلَّى رَكْعَةً وَاحِدَةً تُوتِرُ لَهُ مَا قَدْ صَلَّى " .