Musnad Abu Ya'la al-Mawsili 150:357

Abu al-Fadl said: I heard Sufyan say: She—meaning Aisha—said: "I would have days of fasting from Ramadan [remaining], and I would not be able to fast them until Sha'ban [arrived]." All of it is because she was too shy to fast [them] while the Messenger of Allah (peace and blessings of Allah be upon him) was present, according to what Sufyan explained.