CC Ādi 15.9
prabhu kahe, — ekādaśīte anna nā khāibe
His mother replied, “My dear son, I will give You whatever You ask.” Then the Lord said, “My dear mother, please do not eat grains on the Ekādaśī day.”
From the very beginning of His childhood life Śrī Caitanya Mahāprabhu introduced the system of observing a fast on the Ekādaśī day. 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 dhal, but it is enjoined that a Vaiṣṇava should not even take viṣṇu-prasādam 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-prasādam offered to Viṣṇu, although such prasādam 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.