Ketubot, Chapter Nine, Mishnah Six
This mishnah refers to a situation where the husband exempted his wife from having to swear an oath and he also stated that his heirs may not make her take such an oath. The mishnah asks whether or not the heirs may make her take an oath about her subsequent guardianship over her dead husbands property. This situation would arise because typically the widow would continue to live in her husbands home until she either died or remarried.
1) If she went from her husbands grave to her fathers house, or returned to her father-in-laws house but was not made a guardian, the heirs may not make her swear an oath.
2) But if she was made a guardian the heirs may make her swear an oath in respect of [her administration] during the subsequent period but not in respect of the past.
Section one: If after the death of her husband, she leaves her husbands house and returns to her fathers house, or when she returns to her husbands house she does not act as a guardian over his estate, the inheritors cannot make her swear an oath when she collects her ketubah. This is because her husband specifically exempted her from needing to take such an oath. The mishnah additionally informs us that she does not take an oath over the expenses that she incurred while dealing with the burial of her husband, for if she knew that she would have to take an oath, she might not give her husband a proper burial. (We might have thought that since these costs were incurred after his death, she would have to take an oath about her use of the money). In order to ensure respect for the dead, the mishnah teaches that the inheritors may not make her take an oath that she did not take anything for herself while paying for her husbands funeral.
Section two: If she was made a guardian over her husbands property after his death, then the heirs can make her take an oath about subsequent use of the property but not about past use. The stipulation that her husband made that she should not have to take an oath was only effective over things done in his lifetime and not her management of his affairs after his death. If she was afraid of taking an oath, she could have declined acting as a guardian over the estate.