Agni said:
1-9. Ishall describe the vow (to be performed on the) eleventh (lunar) day that would yield enjoyment and emancipation. Oneshould take restricted food, avoid (eating) flesh and copulation. Heshould not take food on the eleventh day in both the lunar fortnights. When there is a conjunction of the eleventh and twelfth (lunar) days (lord) Hari (Vishnu) is present therein. One gets the merit of doing hundred sacrifices by (such fasting and) taking food on the thirteenth (lunar) day. When a part of the eleventh day exists followed by the twelfth day (if one fasts) and takes food on the thirteenth day one gets the merit of. (doing) hundred sacrifices. Oneshould never eat at the junction of tenth and eleventh (lunar) days. It would confer hell. Oneshould fast on the eleventh day and take food the next day (and say) "O lotus-eyed! I am eating. O The unchanging one! (You) become my refuge". When the asterism pusya rules over the eleventh day of a bright fortnight one is said to get undiminishing benefit. It is known to be destroyer of sins. If the (asterism) Śravana marks the conjunction of the eleventh and twelfth (lunar) days, it is said to be victorious. It confers victory on the votaries. The same in the month of phålguna (March-April) and endowed with the (asterism) pusya it is said to be victorious by pious men. (It is said to be of) rores and crores of virtues. Oneshould worship lord Vishnu on the eleventh day. It confers all good, (makes one) wealthy and begets progeny in this world. It makes him honoured in the world of lord Vishņu.
Summary of chapter 187 of the Agni Purāṇa is as follows:
The rules and extraordinary merits of the Ekādaśī vrata are detailed, establishing it as the crown of all monthly vratas. Agni declares that fasting on both pakṣa-eleventh tithis is obligatory for all Vaiṣṇavas; when Ekādaśī and Dvādaśī fall together, Hari is especially present and the combined upavāsa with pāraṇa on trayodaśī yields merit equal to one hundred yajñas. An Ekādaśī contaminated by daśamī-taint (vedha) must not be observed; doing so causes naraka. Pauṣa-śukla-ekādaśī with the Puṣya-nakṣatra yields akṣaya-phala, and Ekādaśī or Dvādaśī coinciding with Śravaṇa-nakṣatra is called Vijayā-tithi with hundred-fold merit. Viṣṇu-pūjā on Ekādaśī is guaranteed to grant dhana, putra, and ultimately Viṣṇu-loka.