Devibhagavatapurana 9.17.41

9.17.41

dhruvameva labhiṣyāmi rādhābhītiṃ pramocaya |
brahyadeva uvāca |
gṛhāṇa rādhikāmantraṃ dadāmi ṣoḍaśākṣaram ||

Brahmâ said:-- “O Child! I now give you the sixteen lettered Râdhâ mantra to you. By Her Grace you will be dear to Râdhâ as Her life. Râdhikâ will not be able to know anything of your secret dealings. O Fortunate! You will be dear to Govinda like Râdhâ.” Thus saying, Brahmâ, the Creator of the world, gave her the sixteen lettered Râdhâ mantra, stotra, Kavacha and mode of worship and puras’charana and He blessed her. Tulasî, then, engaged herself in worshipping Râdhâ, as directed. By the boon of Brahmâ, Tulasî attained Siddhi (success) like Laksmî. Out of the power of the Siddha mantra, She got her desired boon. She became fortunate in getting various pleasures, hard to be attained in this world. Her mind became quiet. All the toils of Tapasyâ disappeared. When one gets the fruit of one’s labour, all the troubles then transform to happiness. She then finished her food and drink and slept on a beautiful bed strewn with flowers and scented with sandal paste.
Here ends the Seventeenth Chapter of the Ninth Book on the anecdote of Tulasî in S’rî Mad Devî Bhâgavatam of 18,000 verses by Maharsi Veda Vyâsa.