All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
'Abdullaah Al-Makhzoomi said: 'Ibn 'Umar used to perform the Witr prayer with one Rak'ah only, then a man came and asked him about the manner of praying the Witr prayer. Therefore, Ibn 'Umar ordered him to separate the Witr single Rak'ah prayer from other prayers. Then the man said: "I fear that this would be the prohibited manner of performing it." Thereupon, Ibn 'Umar told him: ''If you want to know what Allaah and His Prophet legislated then this is it.' [Ibn Khuzaymah]. Al-Albaani ruled this narration to be authentic."
Many other narrations have been reported in this regard but most of them are weak, so there is no need to mention them. The view of the majority of the scholars is that it is disliked to perform the Witr prayer by one Rak'ah only with nothing before it, but if one prays two Rak'ahs or more before it and then performs one Rak'ah of Witr, then this is the Sunnah of the Prophet . A man came and asked the Prophet about the prayer of the night, and the Prophet replied: "The prayer of the night is in twos and twos, and if one fears that Fajr (dawn) would break before having performed his Witr prayer, then he should finish his night prayers by performing one Witr Rak'ah."
For more benefit, please refer to Fatwa 83303.
Allaah Knows best.