Mudgalapurana 3.18.18

3.18.18

gāyatryāḥ śāpamokṣaṃ vai tato dhyānaṃ japaṃ caret |
brahmacārī śatānyeva pañcakāleṣu vai pṛthak ||

Then he should perform release from the curse of the Gāyatrī, meditation, and recitation; / the student [should recite] hundreds separately at each of the [three] times.