From that great lotus flower, Brahmā was generated, but Brahmā certainly could see nothing but the lotus. Therefore, thinking You to be outside, Lord Brahmā dove into the water and attempted to find the source of the lotus for one hundred years. He could find no trace of You, however, for when a seed fructifies, the original seed cannot be seen.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तत्सम्भवः कविरतोऽन्यदपश्यमान- स्त्वां बीजमात्मनि ततं स्वबहिर्विचिन्त्य । नाविन्ददब्दशतमप्सु निमज्जमानो जातेऽङ्कुरे कथमु होपलभेत बीजम्
tatsambhavaḥ kavirato'nyadapaśyamāna- stvāṃ bījamātmani tataṃ svabahirvicintya | nāvindadabdaśatamapsu nimajjamāno jāte'ṅkure kathamu hopalabheta bījam
SB 7.9.34