Musnad Ahmad 32:686

Abu Kamil and ‘Affan narrated to us, they said: Hammad narrated to us, from Musa ibn Anas—‘Affan said in his narration: Humayd informed us, from Musa ibn Anas ibn Malik, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "You have left behind in Medina men; you have not traveled any distance, nor spent any expenditure, nor crossed any valley, but they are with you in it." They said: "O Messenger of Allah, how can they be with us while they are in Medina?" He said: "The excuse detained them."