Musannaf Ibn Abi Shayba 3:4430

Hushaym narrated to us, he said: Mansur informed us, from Bakr ibn Abd Allah al-Muzani, that Ibn Umar prayed two rak'ahs, then he performed the taslim, then he said: "Bring my she-camel, such-and-such." Then he stood up and performed witr with one rak'ah.