From the very beginning of his childhood life Sri Caitanya Mahaprabhu introduced the system of observing a fast on the ekadasi day. In the bhakti-sandarbha, by Srila Jiva Gosvami, there is a quotation from the Skanda Purana admonishing that a person who eats grains on ekadasi becomes a murderer of his mother, father, brother and spiritual master, and even if he is elevated to a vaikuntha planet, he falls down. On ekadasi, everything is cooked for Visnu, including regular grains and dahi, but it is enjoined that a vaisnava should not even take Visnu-Prasada on ekadasi. It is said that a vaisnava does not accept anything eatable that is not offered to lord visnu, but on ekadasi a vaisnava should not touch even maha-prasada offered to Visnu, although such prasada may be kept for being eaten the next day. It is strictly forbidden for one to accept any kind of grain on ekadasi, even if it is offered to lord Visnu.
Caitanya Caritamrita Adi 15.9