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.