Musnad Ahmad 27:425

Yazid narrated to us, Ibn 'Awn informed us, saying: I wrote to Nafi' asking him: What kept Ibn 'Umar from military expeditions, and regarding the people when they go on expeditions, with what do they call the enemy before they fight them? And may a man charge if he is in the battalion without the permission of his leader? He wrote back to me: Ibn 'Umar used to send his children on expeditions and provide mounts. He used to say: "The best of deeds after prayer is Jihad in the way of Allah the Exalted." Nothing kept Ibn 'Umar from the expedition except the testaments of 'Umar, small children, and much property. "The Messenger of Allah (ﷺ) raided the Banu al-Mustaliq while they were unaware, watering their livestock. He killed their combatants, took their women captive, and acquired Juwayriyyah bint al-Harith." He said: Ibn 'Umar narrated this hadith to me, and he was in that army. They used to call [the enemy] in the early days of Islam. As for a man, he should not charge the battalion except with the permission of his leader.