Sahih Muslim
The Book of Jihad and Expedition (Kitab Al-Jihad wal-Siyar)
Book Number: 019 / Hadith Number: 4467
It has been narrated on the authority of Buraida (who heard the tradition from his father) that the Messenger of Allah (may peace be upon him) conducted nineteen military campaigns and he (actually) fought in eight of them.