This is correct, because the sunnah establishes it: the Prophet, may Allah’s blessings and peace be upon him, said to the woman experiencing irregular bleeding: “Regard yourself as menstruating for six or seven days, according to Allah’s knowledge, then bathe.” (1)
This also accords with reality, for in the case of most women it lasts six or seven days.
His statement, “The shortest period of purity between two menstrual periods is thirteen days,” means that if bleeding comes to her ten days after she became pure, it is not menstruation. Whatever she sees before thirteen days have passed is not menstruation, though it is subject to the rulings of irregular bleeding.
The evidence for this is what has been related from Ali, may Allah be pleased with him: a woman came and said that her waiting period had ended within one month. Ali said to Shurayh, “Rule on her case.” He replied, “If she brings testimony from among her close relatives—people whose religious commitment and character are known—it is accepted; otherwise, it is not.” Ali said, “Qālūn”—meaning, “good” in the Byzantine language. (2)
(1) Narrated by Ahmad, 6:439; Abu Dawud, Book of Purification, chapter: “Whoever says that when menstruation begins, she leaves off prayer,” no. 287; al-Tirmidhi, Chapters on Purification, chapter: “Reports concerning the mustahadah (woman experiencing irregular bleeding) combining the two prayers,” no. 128; and others.
Abu Hatim al-Razi judged the hadith weak. Al-Daraqutni said: “Ibn Aqil alone narrated it, and he is not strong.” Al-Bayhaqi made a similar statement.
I say: Abdullah ibn Muhammad ibn Aqil was described by al-Bukhari as “close to acceptable in hadith.” Al-Dhahabi said: “His hadith is sound; Ahmad and Ishaq relied upon him.” Al-Mughni, 1, no. 3337. Ibn Hajar said: “He is truthful, though there is some weakness in his hadith, and it is said that he changed in his later years.” Al-Taqrib, p. 542. The hadith was authenticated by Ahmad ibn Hanbal, al-Tirmidhi, and al-Nawawi, and graded hasan by al-Bukhari. See: Al-Ilal al-Kabir of al-Tirmidhi, 1:187; Al-Ilal of Ibn Abi Hatim, 1:51, no. 123; Al-Khulasah, no. 632; Al-Talkhis, no. 224.
(2) Narrated by al-Bukhari in suspended form, using a tentative wording, in the Book of Menstruation, chapter: “If a woman menstruates three times in one month.” Ibn Hajar said: “Al-Darimi narrated it with a connected chain, and its narrators are trustworthy.” See: Sunan al-Darimi, no. 854; Fath al-Bari, commentary on the chapter heading of hadith no. 325.