«فَلَمَّا حَلَلْتُ حُلْوَانَ»
Then, when I arrived in Ḥulwān.
Ḥulwān is the town associated with this episode, described in the text's geographical context as lying between Baghdad and Hamadan. Ḥalaltu conveys arriving and taking up residence or stopping there.
The phrase fa-lammā shifts the narrative from habitual intellectual wandering to a particular place and encounter.