Musannaf Ibn Abi Shayba 7:79

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mughira, from Ibrahim, he said: I asked him about a woman who was a heavy sleeper, who slept while her son was with her, and he woke up dead. He said: "It is better for her soul that she expiate by freeing a slave or fasting two consecutive months." I said: "If she menstruates before that, which is inevitable for women, does she make up the days of her menstruation when she finishes?"