Musnad Ahmad 593:5

Waki’ narrated to us, ‘Abd al-Hamid ibn Bahram narrated to us, from Shahr ibn Hawshab, from Ibn Ghanm al-Ash’ari: That the Prophet, peace and blessings of Allah be upon him, said to Abu Bakr and ‘Umar: "If you two agree upon a consultation, I would not oppose you."