From vol. 2 of Conversations, interviews in the early 1980s between Osvaldo Ferrari and Jorge Luis Borges (pg 49-50).
Ferrari: Talking about that – courage seems to be another of her characteristics. One has to remember the telephone calls!
Borges: Yes, she once received a telephone call and a duly coarse, menacing voice said to her, “I’m going to kill you and your son.” “Why, senor?” my mother asked with a rather surprising courtesy. “Because I’m a Peronist.” “Well,” my mother said, “as far as my son is concerned, he leaves the house at 10 every morning. All you have to do is wait for him and kill him. As for me, I’m now (I don’t remember what age she was) 80-something years old – I would advise you not to waste your time talking on the telephone! Because if you don’t hurry, I’ll die before you get to me.” Then the voice put the phone down. I asked her the day after, “Did someone call last night?” “Yes,” she said, “some fool called me at two in the morning,” and then she repeated the conversation. After that there were no more calls. Of course, that nuisance-caller terrorist must have been so shocked that he didn’t dare repeat his offence.