It is stated in the Padma Purana:

bhuyo-bhuyo drdha vani
sryatam syata janah
na bhoktavyam na bhoktavyam
na bhoktavyam harer dine

“O human beings, please listen, I tell you repeatedly with steadfast determination, please never eat any grains on the Day of Lord Hari (Ekadasi day, the Lord’s appearance day like Janmastami day etc.). Please never eat any food grains even by mistake on the Day of Lord Harl. Please do not eat grains on the Day of Lord Hari even if forced or compelled to do so.”

The reason to not eat food grains on Ekadasi is stated in the Vrihan Naradiya Purana thus:

yani kani ca papani
brahma hatyadikani ca
annam asritya tiathanti
samprapte hari vasare

“Every type of sin in this world, including the grievous sin of killing a Brahmana, reside in food grains on the Day of Lord Hari (Ekadasi and other fasting days). If grains are eaten on fasting days sins enter the body of that human being.”

The Hari Bhakti Vilasa states:

brahmacari grhastho va
vanaprastho ‘thava yatih
ekadasyam hi bhunjano
bhukte go-mamsam eva hi

“Whether a person is Brahmachari (unmarried celibate student), Grihastha (householder), Vanaprasthi (retired, elderly), Sanyasi (renounced person) or any other social order, if they eat food grains on Ekadasi, they acquire the same sin as that received by eating cow’s meat.”

The Skanda Purana also states:

matr ha pitr has caiva
bhratr ha guru has tatha
ekadasyas tu ya bhunkte
visnu-lokac cyuto bhavet

“Those who eat grains on Ekadasi and on the Day of Lord Hari (including Janmastami, Gaura Purnima etc.) obtain the sin equal to killing one’s own mother, own father, own brother and own Guru; and they cannot reach the spiritual world, which is why one should never eat grains on fasting days.”

One may think or say that fasting on Ekadasi days and the Lord’s appearance days are meant only for Vaisnava devotees and not for worshippers of Lord Siva or Durga etc. But the Padma Purana informs such people:

na saiva na ca saurohasaun
na sakta gana sevakah
yo bhunkte vasare visnor
jneyah pasvadiko hi sah

“Whether one is a follower of Lord Siva, the goddess Durga, Kali, Surya (sungod), Ganesa, Bhairava or any other demigod, they should avoid eating grains on the fasting days of Lord Hari (Ekadasis and other fasting days of Lord Hari). If one does not rigidly follow this rule and eats grains on fasting days they are considered worse than an animal.”

The Vrihan Naradiya Purana states:

brahma-hatyadi papanam
kathancin niskrtir bhavet
ekadasyat tu yo bhunkte
niskrtir nasti kutracit

“By rigidly following religious and ritualistic processes one may become free from grievous sin like killing a Brahmana, but it is impossible to eliminate the sins of those who eat grains on Ekadasi days.”

It is written in the Skanda Purana that Lord Yamaraja (superintendent of hell) told his messengers not even to go near the immediate three generations of those who fast on Ekadasi days, even if they are lowborn and full of sin. But even if a pious person is learned in the four Vedas and performs rituals and is pure, but eats grains on Ekadasi days, they must be taken to hell to suffer. (The correct Ekadasi days are calculated each year and printed as a calendar and can be gotten upon request). If grain filled prasadam is given to a person on a fasting day it should be kept for consumption on the next day, but should not be consumed on the fasting day. Only fruits, roots (such as potatoes, sweet potatoes, tapioca, sago etc.) water, milk products, nuts, rock salt, Singhara flour, Kutu flower, Sama Rice (found in the Indian grocery stores) and prescribed medicines can be consumed on fasting days. Fasting on Ekadasis began in Satyayuga and everyone followed fasting rules then.

[Bhakti Bhajan Mala Introduction, by Sri Krsna Balaram Swamiji, KBS0046]