First Council of Nicaea, Canon XIII

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.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.