Sahih al-Bukhari 4107
Book 64: Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi) · Narrated by Ibn `Umar
Arabic
حَدَّثَنِي عَبْدَةُ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا عَبْدُ الصَّمَدِ، عَنْ عَبْدِ الرَّحْمَنِ ـ هُوَ ابْنُ عَبْدِ اللَّهِ بْنِ دِينَارٍ ـ عَنْ أَبِيهِ، أَنَّ ابْنَ عُمَرَ ـ رضى الله عنهما ـ قَالَ أَوَّلُ يَوْمٍ شَهِدْتُهُ يَوْمُ الْخَنْدَقِ.
English translation
Narrated Ibn `Umar:The first day (i.e. Ghazwa) I participated in, was the day of Al-Khandaq (i.e. Trench)
Reference: Sahih al-Bukhari 4107, Book of Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi). English translation by Dr. Muhammad Muhsin Khan.