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Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
bukhari:4107 – ʿAbdah b. ʿAbdullāh > ʿAbd al-Ṣamad > ʿAbd al-Raḥman > Ibn ʿAbdullāh b. Dīnār from his father > Ibn ʿUmar

The first day (i.e. Ghazwa) I participated in, was the day of Al-Khandaq (i.e. Trench).  

البخاري:٤١٠٧ – حَدَّثَنِي عَبْدَةُ بْنُ عَبْدِ اللَّهِ حَدَّثَنَا عَبْدُ الصَّمَدِ عَنْ عَبْدِ الرَّحْمَنِ هُوَ ابْنُ عَبْدِ اللَّهِ بْنِ دِينَارٍ عَنْ أَبِيهِ أَنَّ ابْنَ عُمَرَ ؓ قَالَ

أَوَّلُ يَوْمٍ شَهِدْتُهُ يَوْمُ الْخَنْدَقِ