(3) باب لاَ تَقَدَّمُوا رَمَضَانَ بِصَوْمِ يَوْمٍ وَلاَ يَوْمَيْنِ
Abu Huraira (Allah be pleased with him) reported Allah's Messenger (ﷺ) as saying:
Do not observe fast for 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.
Reference: Sahih Muslim 1082 a
In-book reference: Book 13, Hadith 24
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2382