657. Narrated Aboo Huraira (radhi Allaahu anhu) : The Prophet (Sallallaahu Alaihi Wasallam) said,
None of you should observe Saum (fast) for a day or two ahead of Ramadaan, unless he has the habit of observing Saum (Nawaafil), (and if his Saum coincides with that day) then he can observe Saum that day.
[Saheeh Al-Bukhaari, Hadeeth No. 138, Vol. 3]
Source : The Book Of As-Siyaam (the fasting) – Chapter 3 – Not to Observe Saum for a day or two ahead of Ramadaan. Al-Lulu Wal-Marjan, Compiled by : Fuwad Abdul-Baqi, Translated by : Dr. Muhammad Muhsin Khan