Musannaf Ibn Abi Shayba 36:194

Abu Usama narrated to us, he said: Umar bin Hamza informed us, he said: Salim informed me, he said: Ibn Umar informed me that the Messenger of Allah (peace be upon him) said to Umar: "And what makes you know? Perhaps Allah has looked upon the people of Badr and said: 'Do whatever you wish.'"