In the Bhakti-sandarbha, by Śrīla Jīva Gosvāmī, there is a quotation from the Skanda Purāṇa admonishing that a person who eats grains on Ekādaśī becomes a murderer of his mother, father, brother and spiritual master, and even if he is elevated to a Vaikuṇṭha planet, he falls down. On Ekādaśī, everything is cooked for Viṣṇu, including regular grains and dahi, but it is enjoined that a Vaiṣṇava should not even take viṣṇu-prasāda on Ekādaśī. It is said that a Vaiṣṇava does not accept anything eatable that is not offered to Lord Viṣṇu, but on Ekādaśī a Vaiṣṇava should not touch even mahā-prasāda offered to Viṣṇu, although such prasāda may be kept for being eaten the next day. It is strictly forbidden for one to accept any kind of grain on Ekādaśī, even if it is offered to Lord Viṣṇu.
Ref >> Srila Prabhupada Vani.