“Woe to you! Who told you to do that?” They replied, “Our lord ordered us,” referring to Chosroes. The Messenger of Allah ﷺ said, “But my Lord has commanded me to grow my beard and trim my moustache.” ⦗sound hadith⦘
The very wording of this command shows that carrying it out is obligatory: the doer is rewarded, while the one who neglects it is liable to punishment.
Shaving the beard is an act of disobedience
Allah, exalted be He, says: “Whoever disobeys Allah and His Messenger has certainly gone far astray.” ⦗Al-Ahzab 33:36⦘
He also says: “Whoever disobeys Allah and His Messenger—for him is the Fire of Hell, wherein they will remain forever.” ⦗Al-Jinn 72:23⦘
Earlier we cited the Prophet’s ﷺ order to grow the beard; to defy that order is a forbidden act of disobedience. Allah says: “Whatever the Messenger gives you, take it, and whatever he forbids you, abstain from it.” ⦗Al-Hashr 59:7⦘
The Prophet ﷺ likewise said, “Whatever I forbid you, avoid it.” ⦗Agreed upon⦘
Accordingly, the command to grow and maintain the beard entails a prohibition against shaving it completely or trimming it down to the point that it is nearly shaved, for a command to do something necessarily forbids its opposite.