It is reported about Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that when he was asked about the witr, he said: If I have performed the witr before sleeping, and then I intend to pray at night, I make the witr I had performed even by adding one rak'ah, then I keep praying two rak'ahs at a time, and when I have completed my prayer, I perform one rak'ah of witr, because the Prophet (peace and blessings be upon him) ordered that the witr be made the last part of the night prayer.
Imam Ahmad recorded this narration; its chain contains a narrator, Ibn Ishaq, who is a mudallis (one who conceals defects in the chain), and he is thiqah (trustworthy); the remaining narrators of this narration are the narrators of the Sahih.
Ata' bin al-Sa'ib narrated from several students of Sayyiduna Abdullah (may Allah be pleased with him): Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) used to say: When someone performs the witr and then sleeps, and then wakes up, he should cancel his previous witr and join another rak'ah to it, then after that (after completing his voluntary prayers) he should perform the witr.
Imam al-Tabarani recorded this narration in al-Mu'jam al-Kabir; the narrator Ata' bin al-Sa'ib has been criticised on account of his ikhtilat (memory deterioration).
Sayyiduna Thawban (may Allah be pleased with him) narrates: We were travelling with the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "This journey is a source of hardship and burden. When any of you has prayed the Witr, he should pray two rak'ahs. If he wakes up, well and good; otherwise those two will count for him."
Al-Tabarani recorded it in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat; its chain contains Abdullah bin Salih, the scribe of al-Layth, who has been criticized.