Halakha Ulangan 10:8: Mishneh Torah, Prayer and the Priestly Blessing

The Standing position. The Amidah [main prayer] is only recited, standing. A person who is on a ship or in a wagon should, if he is able, stand up for that prayer. If unable, he may read it, sitting in his place. A sick person may read it, even lying down on his side, provided, however, that he can concentrate his mind. A thirsty or hungry person is in the same category as the sick: if he can concentrate, he recites the prayer; if not, he should not do so till he has refreshed himself with food and drink. A person who is riding, need not alight, even if there is some one to hold his beast so that his mind may be at ease.

In the morning service, in the additional service and in the Neilah (closing) service, the priests uplift their hands to pronounce the Priestly Benediction. This is not done in the afternoon service; for in the afternoon [on ordinary days] all have had their meals, and the priests may possibly have drunk wine, and an inebriate is forbidden to perform this rite. Nor even on a fast-day do they pronounce the Priestly Benediction, for fear that if permitted on such a day, the rule might be disregarded in the afternoon service on other days.

Transgressions: A priest who had slain a human being, even if he has repented, must not perform this rite, for it is said, "Your hands are full of blood" (Isaiah 1:15), and it is further written, "And when ye spread forth your hands I will hide mine eyes from you" (Isaiah 1:15). A priest who had worshiped idols, whether under compulsion or in error, even if he had repented, must never perform this rite, for it is said, "Nevertheless, the priests of the high places came not up to the altar of the Lord in Jerusalem" (II Kings 23:9); and the Priestly Benediction is like the Temple service, as it is said, "To minister unto Him and to bless in His name" (Deuteronomy 10:8). So too, a priest who had become a convert to idolatry, though he returned to Judaism, never performs this rite. The commission of other transgressions is no disqualification.