Forbiddence of Fasting on the Day of Doubt
by Shaikh Saleem al-Hilaalee and Shaikh 'Alee Hasan 'Alee 'Abdul-Hameed
A Muslim should not precede Ramadan by fasting a day or two before it, thinking that it might be Ramadan, unless that occurs on a day which he is accustomed to fasting.
From Aboo Hurairah, r.a., who said: He (saaws) said: "Do not pre-empt Ramadan by fasting a day or two before it, except for a man fasting his usual fast - then let him fast it." [Muslim]
So be aware, O brother in Islam, that whoever fasts the day about which there is doubt has disobeyed Allah's Messenger Peace and Blessings of Allah be upon him. Silah ibn Zufar reports from 'Ammar: "Whoever fasts the day about which there is doubt has disobeyed Abul-Qaasim (r.a.)."
[Reported in mu'allaq form by al-Bukhaaree (4/119), connected by Abu Daawood (no. 3334), at-Tirmidhee (no. 686), an-Nasaa'ee (no. 2188) and Ibn Maajah (no. 3334). Its isnaad contains Abu Ishaaq as-Sabee'ee who is mudallis who has performed 'an'anah. His memory also deteriorated at the end of his life, however it has other chains and supports which are quoted by Ibn Hajr in Taghleequt Ta'leeq (3/141-142) which makes the hadeeth hasan]