Musnad Abu Ya'la al-Mawsili 150:408

Suwayd ibn Sa'id narrated to us, 'Abd al-'Aziz ibn Muhammad narrated to us, from Yahya ibn Sa'id, from 'Amrah, from 'Aishah, 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 'Aishah, and she had asked his permission, and Hafsah, and Zaynab. The Prophet, 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.