Srimad Bhagavatam Canto 06, Chapter 16, Text 27

SB 6.16.27

citraketus tu tam vidyam
 yatha narada-bhasitam
dharayam asa saptaham
 ab-bhaksah susamahitah
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Fasting and drinking only water, Citraketu for one week continuously chanted with great care and attention the mantra given by Narada Muni.
Srimad Bhagavatam Canto 06, Chapter 16, Text 26
Srimad Bhagavatam Canto 06, Chapter 16, Text 28