SB 10.13.3
srnusvavahito rajann
api guhyam vadami te
bruyuh snigdhasya sisyasya
guravo guhyam apy uta
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O King, kindly hear me with great attention. Although the activities of the Supreme Lord are very confidential, no ordinary man being able to understand them, I shall speak about them to you, for spiritual masters explain to a submissive disciple even subject matters that are very confidential and difficult to understand.