Milk changes into yogurt when mixed with a yogurt culture, but actually it is constitutionally nothing but milk. Similarly, Govinda, the S P of Godhead, assumes the form of Lord Siva (Sambhu) for the special purpose of material transactions

Expressions researched:
"Milk changes into yogurt when mixed with a yogurt culture, but actually it is constitutionally nothing but milk. Similarly, Govinda, the Supreme Personality of Godhead, assumes the form of Lord Śiva [Śambhu] for the special purpose of material transactions"

Sri Caitanya-caritamrta

CC Madhya-lila

Milk changes into yogurt when mixed with a yogurt culture, but actually it is constitutionally nothing but milk. Similarly, Govinda, the Supreme Personality of Godhead, assumes the form of Lord Śiva [Śambhu] for the special purpose of material transactions. I offer my obeisances at His lotus feet.

CC Madhya 20.310, Translation and Purport

Milk changes into yogurt when mixed with a yogurt culture, but actually it is constitutionally nothing but milk. Similarly, Govinda, the Supreme Personality of Godhead, assumes the form of Lord Śiva [Śambhu] for the special purpose of material transactions. I offer my obeisances at His lotus feet.

This is a quotation from the Brahma-saṁhitā (BS 5.38).