Musnad Ahmad 1153:1143

'Abd al-Rahman narrated to us, from Mu'awiyah, from 'Abdullah ibn Abi Qays, who said: I heard 'Aishah saying: "The Messenger of Allah (peace and blessings of Allah be upon him) used to observe the new moon of Sha'ban with a care that he did not observe for any other. Then, he would fast upon sighting the moon of Ramadan. If it was obscured from him, he would count thirty days, then he would fast."