Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 4

Sanskrit:

मैत्रेय उवाचविरिञ्चोऽपि तथा चक्रे दिव्यं वर्षशतं तप: ।आत्मन्यात्मानमावेश्य यथाह भगवानज: ॥ ४ ॥

ITRANS:

maitreya uvācaviriñco ’pi tathā cakredivyaṁ varṣa-śataṁ tapaḥātmany ātmānam āveśyayathāha bhagavān ajaḥ

Translation:

The greatly learned sage Maitreya said: O Vidura, Brahmā thus engaged himself in penances for one hundred celestial years, as advised by the Personality of Godhead, and applied himself in devotional service to the Lord.

Purport:

That Brahmā engaged himself for the Personality of Godhead, Nārāyaṇa, means that he engaged himself in the service of the Lord; that is the highest penance one can perform for any number of years. There is no retirement from such service, which is eternal and ever encouraging.