And qurʾ denotes blood because it becomes manifest and emerges. It likewise denotes a time, since times are determined by something observable. The interval of purity is then included in the term qurʾ by extension, just as the night is included in the term “day.” The Prophet, may Allah bless him and grant him peace, said to a woman experiencing non-menstrual bleeding:
“Refrain from prayer during the days of your menstrual periods.”
An interval of purity followed by menstruation constitutes a qurʾ; thus, the term encompasses the whole. An interval of purity alone, however, is not called a qurʾ. Hence, if a woman is divorced during a menstrual period, she does not count that as a qurʾ, because she must observe a waiting period of three qurūʾ. If she is divorced during an interval of purity, the qurʾ consists of the menstrual period together with the preceding interval of purity. This is why the senior Companions—including ʿUmar, ʿUthmān, ʿAlī, Abū Mūsā, and others—held that the qurūʾ are menstrual periods: she is commanded to wait for three qurūʾ. If a qurʾ meant an interval of purity, her waiting period would consist of two qurūʾ and part of a third.
The disagreement between the two groups concerns the third menstrual period. The senior Companions and those who agreed with them say that her husband has the prior right to take her back as long as she has not bathed after her third menstrual period. The younger Companions say that once she enters her third menstrual period, she is free to remarry.
It is established by both scriptural evidence and consensus that the sunnah is to divorce her during an interval of purity in which he has not had intercourse with her. By then, part of that interval of purity has already elapsed. Yet Allah commanded that divorce take place with the waiting period still ahead, not when it is already under way. His words “three qurūʾ” specify a number, unlike the word “months,” which is a plural form, not a number. There must therefore be three qurūʾ, as Allah commanded; part of the third is not sufficient.