Garudapurana 3.14.34

3.14.34

niḥ sāraṃ manujendrāṇāmiti vedavidāṃ matam /
āṣāḍhamāse garuḍa śāko niḥ sāra ucyate //

[for eating it is] devoid of essence for the lords of men: such is the view of the knowers of the Veda. / In the month of Āṣāḍha, O Garuḍa, leafy vegetables are said to be devoid of essence.