What is the authenticity of the narration which states that the slave women of 'Umar used to serve with their hair uncovered and their breasts jiggling?

The narration reported by Al-Bayhaqi from Anas ibn Malik, that 'Umar's bondswomen used to serve with their hair uncovered and their breasts visible, has a hasan (good) chain of transmission. Its meaning is that their hair would brush against their chests due to the speed of their movement while serving.

The Sharia has differentiated between free women and bondswomen in terms of hijab. A free woman observes a complete hijab, whereas a bondswoman is permitted to uncover her head, hands, and face due to her need for service and because people are not as inclined to desire them. It was reported that 'Umar (may Allah be pleased with him) used to strike a bondswoman who veiled herself, saying, "Are you imitating free women?"

However, if there is a fear of fitna (temptation) from the bondswoman or if corrupt people are numerous, then it becomes obligatory for the bondswoman to observe hijab, and it becomes obligatory to lower the gaze from her. The ruling in this matter varies according to time, place, and the presence or absence of fitna.