Hadith: A man who had drunk alcohol was brought to the Prophet (may Allah's peace and blessings be upon him) who hit him with a palm stalk with about forty lashes.

عن أنس بن مالك رضي الله عنه «أَنَ النّبي صلى الله عليه وسلم أُتِي بِرَجُل قَدْ شَرِب الْخَمْرَ، فَجَلَدَهُ بِجَريدة نحو أربعين»
[صحيح] - [متفق عليه]

Anas ibn Mālik (may Allah be pleased with him) reported: A man who had drunk alcohol was brought to the Prophet (may Allah's peace and blessings be upon him), who hit him with a palm stalk with about forty lashes.
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

A man had drunk alcohol during the time of Prophet (may Allah's peace and blessings be upon him), who hit him with a palm stalk with about forty lashes as a legal punishment for drinking alcohol. In his reign, Abu Bakr (may Allah be pleased with him) applied the same number of lashes to a drunkard. When ‘Umar (may Allah be pleased with him) became a Caliph and the Muslims conquered several countries and mingled with others, drinking alcohol became widespread. So, ‘Umar (may Allah be pleased with him), as usual, consulted the scholars of the Companions on what punishment to apply on the drunkards to deter them from committing that sin. It was ‘Umar's habit to seek counsel on matters of importance that required personal legal reasoning. As the number of drunkards increased during the reign of ‘Umar, ‘Abdur-Rahmān ibn ‘Awf (may Allah be pleased with him) advised ‘Umar to increase the punishment of drunkards to eighty lashes so that it would become similar to the next lightest prescribed punishment, which is applicable to slanderers. So, ‘Umar (may Allah be pleased with him) increased the punishment of drunkards to eighty lashes, forty as a prescribed legal punishment, and forty as an additional discretionary punishment left to the discretion of the ruler (or judge), when necessary.