SB 5.18.6
vedan yugante tamasa tiraskrtan
rasatalad yo nr-turanga-vigrahah
pratyadade vai kavaye ’bhiyacate
tasmai namas te ’vitathehitaya iti
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
At the end of the millennium, ignorance personified assumed the form of a demon, stole all the Vedas and took them down to the planet of Rasatala. The Supreme Lord, however, in His form of Hayagriva retrieved the Vedas and returned them to Lord Brahma when he begged for them. I offer my respectful obeisances unto the Supreme Lord, whose determination never fails.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Although Vedic knowledge is imperishable, within this material world it is sometimes manifest and sometimes not. When the people of this material world become too absorbed in ignorance, the Vedic knowledge disappears. Lord Hayagriva or Lord Matsya, however, always protects the Vedic knowledge, and in due course of time it is again distributed through the medium of Lord Brahma. Brahma is the trustworthy representative of the Supreme Lord. Therefore when he again asked for the treasure of Vedic knowledge, the Lord fulfilled his desire.