Imran bin Husayn reported,

Imran reported from the Prophet ﷺ that he asked him, or asked a man while Imran was listening, "So-and-so, did you not fast during the last days of this month?" The narrator said, I think he meant Ramadan.The man replied, "No, Messenger of Allah."He said, "Then when you break your fast, fast two days."

Azami 4427

Sahih

عن عمران بن حصين

عن النبي ﷺ أنه سأله أو سأل رجلا وعمران يسمع، فقال: يا فلان، أما صمت سرر هذا الشهر؟ قال: أظنه قال يعني رمضان، قال الرجل: لا، يا رسول الله. قال: فإذا أفطرت فصم يومين.

الأعظمي 4427صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Fasting
Chapter
Voluntary fasting and encouragement to do it
Section
Fasting the last days of Shaban
Numbering
Jami al-Kamil 4427 (2nd edition)
Azami 4427