The Messenger of Allah ﷺ said: "Prayer at night is two by two, then when you want to finish, pray one rak'h which will make the total number that you prayed odd." (Using translation from Nasāʾī 1692)   

عَنْ رَسُولِ اللهِ ﷺ قَالَ «صَلَاةُ اللَّيْلِ مَثْنَى مَثْنَى فَإِذَا أَرَدْتَ أَنْ تَنْصَرِفَ فَارْكَعْ رَكْعَةً وَاحِدَةً تُوتِرُ لَكَ مَا صَلَّيْتَ»  

The Prophet ﷺ used to pray eleven rak'ahs at night, ending them with one rak'ah of witr, then he would lie down on his right side. (Using translation from Nasāʾī 1696)  

أَنَّ النَّبِيَّ ﷺ كَانَ «يُصَلِّي مِنَ اللَّيْلِ إِحْدَى عَشْرَةَ رَكْعَةً يُوتِرُ مِنْهَا بِوَاحِدَةٍ ثُمَّ يَضْطَجِعُ عَلَى شِقِّهِ الْأَيْمَنِ»