Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 27
Sanskrit:
चित्रकेतुस्तु तां विद्यां यथा नारदभाषिताम् ।धारयामास सप्ताहमब्भक्ष: सुसमाहित: ॥ २७ ॥
ITRANS:
citraketus tu tāṁ vidyāṁyathā nārada-bhāṣitāmdhārayām āsa saptāhamab-bhakṣaḥ susamāhitaḥ
Translation:
Fasting and drinking only water, Citraketu for one week continuously chanted with great care and attention the mantra given by Nārada Muni.