فلم يكن إلا كلمح البصر أو هو أقرب حتى أنشد فأغرب
It happened in no more than the twinkling of an eye, or even less, until he recited and astonished his audience.
The recognizable expression kalamḥ al-baṣar aw huwa aqrab comes from the Quranic wording about the swiftness of the Hour (Qur'an 16:77). In this prose sentence it expresses the speed of a particular incident.
It is Quranic incorporation in prose: the wording is recognizable, while the immediate topic of the new passage differs.