Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 285
Narrated Abu Ishaq:
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."