Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) said: Abu Bakr (may Allah be pleased with him) wrote to Amr bin al-As that the Messenger of Allah (peace and blessings be upon him) used to consult others in matters of war, so you too should adopt this way.
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 46