The Prophet ﷺ said: Prayer by night and day should consist of pairs of rak'ahs.  

عَنِ النَّبِيِّ ﷺ قَالَ صَلاَةُ اللَّيْلِ وَالنَّهَارِ مَثْنَى مَثْنَى  

The Prophet ﷺ said: Prayer is to be offered in two rak'ahs; and you should recite the tashahhud at the end of two rak'ahs, and express your distress and humility and raise your hands and say praying: O Allah, O Allah. He who does not do so does not offer a perfect prayer. Abu Dawud was asked about offering prayer at night in two rak'ahs. He said: They may be two if you like and four if you like.  

عَنِ النَّبِيِّ ﷺ قَالَ الصَّلاَةُ مَثْنَى مَثْنَى أَنْ تَشَهَّدَ فِي كُلِّ رَكْعَتَيْنِ وَأَنْ تَبَاءَسَ وَتَمَسْكَنَ وَتُقْنِعَ بِيَدَيْكَ وَتَقُولَ اللَّهُمَّ اللَّهُمَّ فَمَنْ لَمْ يَفْعَلْ ذَلِكَ فَهِيَ خِدَاجٌ سُئِلَ أَبُو دَاوُدَ عَنْ صَلاَةِ اللَّيْلِ مَثْنَى قَالَ إِنْ شِئْتَ مَثْنَى وَإِنْ شِئْتَ أَرْبَعًا