The First Council of Nicaea (325 AD)

Concerning the departing, the ancient canonical law is still to be maintained, to wit, that, if any man be at the point of death, he must not be deprived of the last and most indispensable Viaticum. But, if any one should be restored to health again who has received the communion when his life was despaired of, let him remain among those who communicate in prayers only. But in general, and in the case of any dying person whatsoever asking to receive the Eucharist, let the Bishop, after examination made, give it him.

Ancient Epitome of Canon XIII.

The dying are to be communicated. But if any such get well, he must be placed in the number of those who share in the prayers, and with these only.

It cannot be denied that antiquity used the name “Viaticum” not only to denote the Eucharist which was given to the dying, but also to denote the reconciliation, and imposition of penance, and in general, everything that could be conducive to the happy death of the person concerned, and this has been shown by Aubespine. But while this is so, the more usual sense of the word is the Eucharist. For this cannot be denied that the faithful of the first ages of the Church looked upon the Eucharist as the complement of Christian perfection, and as the last seal of hope and salvation. It was for this reason that at the beginning of life, after baptism and confirmation, the Eucharist was given even to infants, and at the close of life the Eucharist followed reconciliation and extreme unction, so that properly and literally it could be styled “the last Viaticum.” Moreover for penitents it was considered especially necessary that through it they might return to the peace of the Church; for perfect peace is given by that very communion of the Eucharist. [A number of instances are then cited, and various ancient versions of the canon.] Balsamon and Zonaras also understand the canon as I have done, as is evident from their commentaries, and so did Josephus Ægyptius, who in his Arabic Paraphrase gives the canon this title: “Concerning him who is excommunicated and has committed some deadly sin, and desires the Eucharist to be granted to him.”

This canon is found in the Corpus Juris Canonici, Gratian, Decretum Pars. II. causa xxvi, Quæs. VI., c. ix.