Fasting here signifies voluntary fasting. Similar is the case of other voluntary prayers, i.e., voluntary Salat, recitation, etc. It is not permissible to do such acts without the permission of the husband if he is present. It should be borne in mind that a women is not permitted to let even her natural relations (who are Mahram) come in her home in the absence of her husband. She can, however, allow such persons to come about whom her husband has given express permission or in respect of whom he keeps silent.