SB 4.8.30

atha mātropadiṣṭena
 yogenāvarurutsasi
yat-prasādaṁ sa vai puṁsāṁ
 durārādhyo mato mama
Synonyms: 
atha — therefore; mātrā — by your mother; upadiṣṭena — being instructed; yogena — by mystic meditation; avarurutsasi — want to elevate yourself; yat-prasādam — whose mercy; saḥ — that; vai — certainly; puṁsām — of the living entities; durārādhyaḥ — very difficult to perform; mataḥ — opinion; mama — my.
Translation: 
Now you have decided to undertake the mystic process of meditation under the instruction of your mother, just to achieve the mercy of the Lord, but in my opinion such austerities are not possible for any ordinary man. It is very difficult to satisfy the Supreme Personality of Godhead.
Purport: 

The process of bhakti-yoga is simultaneously very difficult and very easy to perform. Śrī Nārada Muni, the supreme spiritual master, is testing Dhruva Mahārāja to see how determined he is to prosecute devotional service. This is the process of accepting a disciple. The great sage Nārada has come to Dhruva under the direction of the Supreme Personality of Godhead just to initiate him, yet he is testing Dhruva’s determination to execute the process. It is a fact, however, that for a sincere person devotional service is very easy. But for one who is not determined and sincere, this process is very difficult.