Hadith: Do not fast a day or two days ahead of Ramadan, except a person who is in the habit of observing a particular fast; he may fast on that day

عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:

«لَا تَقَدَّمُوا رَمَضَانَ بِصَوْمِ يَوْمٍ وَلَا يَوْمَيْنِ إِلَّا رَجُلٌ كَانَ يَصُومُ صَوْمًا فَلْيَصُمْهُ».
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "Do not fast a day or two days ahead of Ramadan, except a person who is in the habit of observing a particular fast; he may fast on that day.”
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

In this Hadīth, Abu Hurayrah (may Allah be pleased with him) reports that the Prophet (may Allah's peace and blessings be upon him) forbade fasting one or two days before the beginning of the month of Ramadan, except for a person who normally fasts on specific days such as Mondays, for example. If that day is one or two days before the start of Ramadan, then, in this case, he may fast on it, because he did not introduce thereby a non-ordained extra act of worship.