The Messenger of Allah ﷺ used to be more careful in tracking the days of Shaban than he was with any other month. He would then begin fasting upon sighting the crescent of Ramadan, and if the sky was overcast, he would count thirty days and then begin fasting.
حدثنا أحمد بن حنبل حدثني عبد الرحمن بن مهدي حدثني معاوية بن صالح عن عبد الله بن أبي قيس قال سمعت
كان رسول الله ﷺ يتحفظ من شعبان ما لا يتحفظ من غيره ثم يصوم لرؤية رمضان فإن غم عليه عد ثلاثين يوما ثم صام
Its isnad is sahih. Ibn Abdul-Barr reported it in Tamhid (14/353) from the hadith of Abu Dawud, through him, and it is found in Musnad Imam Ahmad (6/149). Ibn Khuzayma graded it sahih (no. 1910), as did Ibn Hibban (no. 869), and Hakim graded it as meeting the standard of the two Shaykhs (1/423), and Dhahabi agreed with him.