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SB 6.16.27

citraketus tu tāṁ vidyāṁ
 yathā nārada-bhāṣitām
dhārayām āsa saptāham
 ab-bhakṣaḥ susamāhitaḥ
Synonyms: 
citraketuḥ — King Citraketu; tu — indeed; tām — that; vidyām — transcendental knowledge; yathā — just as; nārada-bhāṣitām — instructed by the great sage Nārada; dhārayām āsa — chanted; sapta-aham — continuously for one week; ap-bhakṣaḥ — only drinking water; su-samāhitaḥ — with great attention and care.
Translation: 
Fasting and drinking only water, Citraketu for one week continuously chanted with great care and attention the mantra given by Nārada Muni.