Musannaf Ibn Abi Shayba 7:304

'Abd al-'Aziz ibn Muhammad al-Darawardi narrated to us, from Musa ibn 'Uqbah, that a woman from his family had a vow of one month of i'tikaf in the mosque. She performed i'tikaf for twenty-nine days, then she menstruated, so she returned to her family. Then she became pure, and her husband had intercourse with her. He said: I came to Salim and al-Qasim, and they both said: "Go to Sa'id ibn al-Musayyib, then come back to us." He said: So I went to Sa'id and asked him. He said: "He has committed a violation of the limits of Allah and has erred against the Sunnah, and she must restart [the i'tikaf]." He said: So I returned to al-Qasim and Salim and informed them of what he had said. They both said: "That is our opinion."