Abu Mansur (may Allah have mercy on him), the freed slave of Sa'd bin Abi Waqqas, said: I asked Abdullah bin Umar (may Allah be pleased with them both) about the witr of the night, and Ibn Umar (may Allah be pleased with them both) said: "My son! Do you know the witr of the day?" I said: "Yes, it is Maghrib." Ibn Umar (may Allah be pleased with them both) said: "You have spoken the truth; and the witr of the night is only one rak'ah, which the Messenger of Allah (peace and blessings be upon him) commanded." I said: "O Abu Abd al-Rahman! People call it al-butayra' (the cut-off prayer)." He said: "My son! That is not al-butayra'. Al-butayra' is when a man prays one rak'ah complete in its bowing, prostration and standing, then stands for the next rak'ah and completes neither its bowing and prostration nor its standing." Among them is Abdullah bin Abbas bin Abd al-Muttalib (may Allah be pleased with them both).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4791