Musnad Abu Ya'la al-Mawsili 150:2

Suwayd narrated to us, Abd al-Aziz—meaning al-Darawardi—narrated to us, from Yahya ibn Sa'id, from Amrah, from Aisha, who said: When the Prophet, peace and blessings of Allah be upon him, intended to perform i'tikaf, he would pray the morning prayer, then enter his place of i'tikaf. When it was the morning of the twenty-first, he departed from the morning prayer and entered the mosque. He saw tents—the tent of Aisha, and she had asked his permission, and [the tent of] Zaynab. The Messenger of Allah, peace and blessings of Allah be upon him, said, "Is it righteousness that you intend by them?" So he delayed his i'tikaf until Shawwal.