Observing that people in general were diminished in their life span, strength and intelligence by the influence of time, great sages took inspiration from the Personality of Godhead sitting within their hearts and systematically divided the Vedas.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
क्षीणायुषः क्षीणसत्त्वान् दुर्मेधान् वीक्ष्य कालतः । वेदान् ब्रह्मर्षयो व्यस्यन् हृदिस्थाच्युतचोदिताः
kṣīṇāyuṣaḥ kṣīṇasattvān durmedhān vīkṣya kālataḥ | vedān brahmarṣayo vyasyan hṛdisthācyutacoditāḥ
SB 12.6.47