It is permissible to fast on the Day of Doubt with the intention of a voluntary fast, and it is not disliked. If it becomes clear that it is Ramadan, then the fast counts for Ramadan. If it becomes clear that it is Sha'ban, then it counts for Sha'ban. If he invalidates his fast, he must make it up because he has started it with commitment. Aisha (may Allah be pleased with her) said: "I never saw him fasting in any month more than he did in Sha'ban; he used to fast all of Sha'ban, except for a little." This is reported in Sahih Muslim 2: 811, and Sahih Ibn Hibban 8: 404. As for what was narrated from Abu Huraira (may Allah be pleased with him) that the Messenger of Allah (peace be upon him) said: "When half of Sha'ban has passed, do not fast," this is found in Sunan Abu Dawood 2: 300, Sunan an-Nasa'i 2: 172, and Jami' at-Tirmidhi 3: 115, and he said it is Hasan Sahih. Abu Zur'ah said it is weak, as mentioned in the Questions of Al-Baradh'i 1: 388. Ibn Hajar said in Fath al-Bari: It was narrated by the authors of the Sunnah and authenticated by Ibn Hibban and others... The majority of scholars said it is permissible to fast voluntarily after the middle of Sha'ban and they weakened the hadith regarding it. Ahmad and Ibn Ma'in said it is weak. What is the ruling on fasting on the Day of Doubt with the intention of an obligatory fast? It is disliked to fast on the Day of Doubt with an intention other than voluntary, such as fasting it with the intention of an obligatory fast. It counts for what he intended if it becomes clear that it is from Sha'ban. If it turns out that it is from Ramadan, then his intention counts for Ramadan if he is a resident; because if a traveler intended another obligatory fast, it would count for what he intended. Abu Huraira (may Allah be pleased with him) reported that the Messenger of Allah (peace be upon him) said: "Do not precede Ramadan by fasting a day or two, except for a man who was accustomed to fasting, let him fast it." This is found in Sahih Muslim 2: 762, Sahih al-Bukhari 2: 676, Sahih Ibn Hibban 8: 358, and Musnad al-Mustakhraj 3: 160.