Musannaf Ibn Abi Shayba 7:312

Ibn Mushir narrated to us, from Isma'il bin Abi Khalid, from al-Sha'bi, regarding a woman who vowed to seclude herself (i'tikaf) for fifty days, so she secluded herself for forty days. Then her husband came and sent for her, and she came to him. He said: She shall complete what remains.