(1) باب غَزْوَةِ الْعُشَيْرَةِ أَوِ الْعُسَيْرَةِ
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: Sahih al-Bukhari 3949
In-book reference: Book 64, Hadith 1
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 5, Book 59, Hadith 285