It is narrated from Hasan al-Basri (may Allah have mercy on him), he said: If possible, do not look at the hair of any of your household except your wife and young daughters.
Hadith Referenceالادب المفرد / كتاب الصغير / 366
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Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: The intention of Hasan al-Basri rahimahullah is that those women among relatives who are non-mahram, even if they are related by blood, their hair should not be seen, so that one may be protected from falling into temptation. However, if it is a small girl, then that is a different matter. Or, it may be intended that even young sisters and daughters should not be seen with their heads uncovered.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 366