Musnad Ahmad 961:59

Abu al-Mughirah and Abu al-Yaman said: Abu Bakr narrated to us, al-Walid ibn Sufyan ibn Abi Maryam narrated to me, from Yazid ibn Qutayb al-Sakuni, from Abu Bahriyyah, who said—Abu al-Mughirah said in his narration—from Abdullah ibn Qays, who said: I heard Mu'adh ibn Jabal say: The Messenger of Allah, peace and blessings of Allah be upon him, said: "The Great Slaughter (al-Malhamah al-Uzma), the conquest of Constantinople, and the emergence of the Dajjal [will occur] within seven months."