It is forbidden to fast on the day of doubt if one is certain of their intention for Ramadan: like saying: "I intend to fast tomorrow for the first day of Ramadan this year"; due to the hadith of Ammar (may Allah be pleased with him): "Whoever fasts on the day of doubt has disobeyed Abu al-Qasim," in Sahih al-Bukhari 2: 674 (commented), Al-Mustadrak 1: 585, and Jami` at-Tirmidhi 3: 70, and he said: "Hasan Sahih." And from Abu Huraira (may Allah be pleased with him), the Messenger of Allah (peace be upon him) said: "Do not precede Ramadan by fasting a day or two, except for a man who used to fast, so let him fast it," in Sahih Muslim 2: 762, Sahih al-Bukhari 2: 676, Sahih Ibn Hibban 8: 358, and Al-Musnad Al-Mustakhraj 3: 160.