كتاب الوتر
Read the English translation first, with the original Arabic directly underneath.
Narrated `Abdullah bin `Umar:
The Prophet (ﷺ) said, "Night prayer is offered as two rak`at followed by two rak`at and so on, and if you want to finish it, pray only one rak`a which will be witr for all the previous rak`at." Al-Qasim said, "Since we attained the age of puberty we have seen some people offering a three-rak`at prayer as witr and all that is permissible. I hope there will be no harm in it."
صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى، فَإِذَا أَرَدْتَ أَنْ تَنْصَرِفَ فَارْكَعْ رَكْعَةً تُوتِرُ لَكَ مَا صَلَّيْتَ