Surah 37. As-Saffat, Ayah 49

This is an ancient Arabian figure of speech derived from the habit of the female ostrich, which buries its eggs in the sand for protection (Zamakhshari). Its particular application to the women who attain to paradise becomes clear from 56:34 ff., which states that all righteous women, irrespective of their age and condition at the time of death, will be resurrected as beautiful maidens.

 ‘Baiḍun maknûn’ can also mean spotless eggs.

This is usually understood to refer to the delicate complexion of a beautiful woman, which is compared to the transparent shell of eggs in the nest, closely guarded by the mother-bird; the shell is warm and free from stain. In Iv. 58 the phrase used is "like rubies and coral", referring to the red or pink of a beautiful complexion.