O my son, the original poet, Brahmā, after mature meditation for one thousand celestial years, could know only that the glories of the Supreme Soul are inconceivable.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
आत्मनोऽवसितो वत्स महिमा कविनाऽऽदिना । संवत्सरसहस्रान्ते धिया योगविपक्वया
ātmano'vasito vatsa mahimā kavinā''dinā | saṃvatsarasahasrānte dhiyā yogavipakvayā
SB 3.6.38