CC Madhya 20.3

ethā gauḍe sanātana āche bandi-śāle
śrī-rūpa-gosāñīra patrī āila hena-kāle
Word for word: 
ethā — here; gauḍe — in Bengal; sanātana — Sanātana Gosvāmī; āche — was; bandi-śāle — in prison; śrī-rūpa-gosāñīra — of Śrīla Rūpa Gosvāmī; patrī — the letter; āila — came; hena-kāle — at that time.
Translation: 
While Sanātana Gosvāmī was imprisoned in Bengal, a letter arrived from Śrīla Rūpa Gosvāmī.
Purport: 

Śrīla Bhaktivinoda Ṭhākura informs us that this letter from Rūpa Gosvāmī to Sanātana Gosvāmī is mentioned by the annotator of the Udbhaṭa-candrikā. Śrīla Rūpa Gosvāmī wrote a note to Sanātana Gosvāmī from Bāklā. This note indicated that Śrī Caitanya Mahāprabhu was coming to Mathurā, and it stated:

yadu-pateḥ kva gatā mathurā-purī
raghu-pateḥ kva gatottara-kośalā
iti vicintya kuruṣva manaḥ sthiraṁ
na sad idaṁ jagad ity avadhāraya

“Where has the Mathurā-purī of Yadupati gone? Where has the Northern Kośalā of Raghupati gone? By reflection, make the mind steady, thinking, ‘This universe is not eternal.’ ”