Masruq states: I visited Sayyidah Aishah al-Siddiqah (may Allah be pleased with her) on a day about which there was doubt whether it was part of Ramadan (or not). Sayyidah Aishah (may Allah be pleased with her) said: "O slave girl, mix some barley drink (sawiq) for him." I said: "I am fasting." Sayyidah Aishah (may Allah be pleased with her) said: "Have you fasted before the month has begun?" I said: "No, but I fasted the whole of Sha'ban, so I fasted this day as well." Sayyidah Aishah (may Allah be pleased with her) said: "Some people fasted before the month (of Ramadan) and before the Prophet (peace and blessings be upon him), so this verse was revealed: 'O you who believe, do not try to go ahead of Allah and His Messenger.'"
Al-Tabarani recorded it in al-Mu'jam al-Awsat. Its chain contains a narrator, Hibban bin Ruqaydah, who is majhul (unknown).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 4827