Hindu Media Wiki

It is said in the PurAnas that the personification of all sins or the pApa purusha resides within the grains like rice on this day. That's why eating all grains including rice is prohibited on the EkAdashi Tithis.

The Kamakoti.com website quotes the Vishnu PurAna in this context as follows:

yAni kAni cha pApAni brahmahatyAdikAni cha | annamAshritya tiShThanti saMprApte harivAsare | aghaM sa kevalaM bhu~Nkte yo bhu~Nkte harivAsare || [viShNupurANe]

Greatest of sins like brahmahatyA are present in anna or food on the eleventh day of every pakSha. One who eats on this day, ingests these sins and attains great misery

This website quotes the Padma PurAna and gives the detailed story of how the pApa purusha got grains as his residence for the EkAdashi Tithis.

The story is quite long. So, i'm not quoting it. In short, Lord Vishnu said to the pApa purusha to make the grains his residence on the EkAdashi Tithis.

[pApa purusha said] Out of fear of Sri Ekadasi I have fled and taken shelter of men; animals; insects; hills; trees; moving and non-moving living entities; rivers; oceans; forests; heavenly, earthly and hellish planetary systems; demigods; and the Gandharvas. I cannot find a place where I can be free from the fear of Sri Ekadasi. O my Master! I am a product of Your creation, so therefore very mercifully direct me to a place where I can reside fearlessly.’”

Vyasadeva then said to Jaimini, “After saying this, the embodiment of all sinful activities (Papa-purusha) fell down at the feet of the Supreme Lord Vishnu, who is the destroyer of all miseries and began to cry.

“After this, Lord Vishnu, observing the condition of the Papa-purusha, with laughter began to speak thus: ‘O Papa-purusha, rise up! Don’t lament any longer. Just listen, and I’ll tell you where you can stay on the lunar day of Ekadasi. On the date of Sri Ekadasi, which is the benefactor of the three planetary systems, you can take shelter of foodstuffs in the form of grains. There is no reason to worry about this any more, because My form as Sri Ekadasi will no longer impede you.’ After giving direction to the Papa-purusha, the Supreme Lord Vishnu disappeared and the Papa-purusha returned to the performance of his own activities.

“Therefore, those persons who are serious about the ultimate benefit of the soul will never eat grains on Ekadasi.