It is narrated from the students of Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that the fatwa of Sayyiduna Ibn Mas'ud (may Allah be pleased with him) was this: when a woman is divorced and people think that her menstruation has ceased, but this is not clear, she should wait for one year. If she does not menstruate during that time, then after the year she observes a waiting period of three months. If she menstruates during those three months, she observes the waiting period by menstruation; and if she (then) menstruates but that menstruation is not completed after she has observed those three months that followed the year, he said: "Do not be hasty concerning her until it is known whether her menstruation has been completed or not."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9588