(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. (ES)
Reference (ES): Sahih Muslim 1082 a (ES)
In-book reference (ES): Book 13, Hadith 24 (ES)
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2382 (ES)