Musnad Ahmad 1153:1879

Ya'la ibn 'Ubayd narrated to us, he said: Yahya narrated to us, from 'Amrah, from 'Aishah, she said: When the Messenger of Allah (peace and blessings of Allah be upon him) wanted to perform I'tikaf, he would pray the morning prayer, then enter the place in which he wanted to perform I'tikaf. He wanted to perform I'tikaf during the last ten days of Ramadan, so he gave an order and a tent was pitched for him. 'Aishah gave an order and a tent was pitched for her, and Hafsah gave an order and a tent was pitched for her. When Zaynab saw their two tents, she gave an order and a tent was pitched for her. When the Messenger of Allah (peace and blessings of Allah be upon him) saw that, he said: "Is it righteousness that you desire?" So he did not perform I'tikaf in Ramadan, and he performed I'tikaf for ten days in Shawwal.