For the first month Dhruva Mahārāja ate only fruits and berries on every third day, only to keep his body and soul together, and in this way he progressed in his worship of the Supreme Personality of Godhead.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
त्रिरात्रान्ते त्रिरात्रान्ते कपित्थबदराशनः । आत्मवृत्त्यनुसारेण मासं निन्येऽर्चयन् हरिम्
trirātrānte trirātrānte kapitthabadarāśanaḥ | ātmavṛttyanusāreṇa māsaṃ ninye'rcayan harim
SB 4.8.72