Śrīla Bhaktivinoda Ṭhākura informs us that this letter from Rūpa Gosvāmī to Sanātana Gosvāmī is mentioned by the annotator of 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 kurusva 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.' "