From Sahih Bukhari (Hadith 3949):
Narrated Abu 'Is-haq:Once, while I was sitting beside Zaid bin Al-Arqam, he was asked, "How many Ghazwat did the Prophet undertake?" Zaid replied, "Nineteen." They said, "In how many Ghazwat did you join him?" He replied, "Seventeen." I asked, "Which of these was the first?" He replied, "Al-`Ashira or Al- `Ashiru
Reference: Bukhari 3949