A man called out to the Messenger of Allah ﷺ while he was in the mosque, asking, "Messenger of Allah, how do I make my night prayer odd-numbered?"The Messenger of Allah ﷺ replied, "Whoever prays should pray two units at a time. If he senses that dawn is approaching, let him perform a single prostration, and that will make all that he has prayed odd-numbered."
أن رجلا نادى رسول الله ﷺ وهو في المسجد، فقال: يا رسول الله! كيف أوتر صلاة الليل؟ فقال رسول الله ﷺ: من صلى فليصل مثنى مثنى، فإن أحس أن يصبح سجد سجدة فأوترت له ما صلى.
Muslim narrated it in the Book of the Prayer of Travelers (749), from the hadith of Abu Usama, from Walid bin Kathir, who said, "Ubaydullah bin Abdullah bin Umar told me that Ibn Umar narrated it to them," and he mentioned it.