Muwatta Malik, Book 18, Hadith 55 (USC-MSA) ⓘ

Yahya related to me from Malik that he had heard the people of knowledge telling people not to fast on the day in Shaban when there was doubt (about whether it was Shaban or Ramadan), if they intended by it the fast of Ramadan. They considered that whoever fasted on that day without having seen (the new moon) had to make up that day if it later became clear that it was part of Ramadan. They did not see any harm in voluntary fasting on that day. Malik said, "This is what we do, and what I have seen the people of knowledge in our city doing."