SB 3.10.4
maitreya uvaca
viriñco ’pi tatha cakre
divyam varsa-satam tapah
atmany atmanam avesya
yathaha bhagavan ajah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The greatly learned sage Maitreya said: O Vidura, Brahma 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 by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
That Brahma engaged himself for the Personality of Godhead, Narayana, 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.