Musnad Ahmad 30:1861

'Affan narrated to us, 'Abd al-Wahid ibn Ziyad narrated to us, 'Amarah ibn al-Qa'qa' narrated to us, Abu Zur'ah narrated to us that he heard Abu Hurayrah say: The Messenger of Allah (may Allah bless him and grant him peace) said: "By the One in Whose hand is my soul, were it not that it would be difficult for my nation, I would not remain behind any expedition that fights in the way of Allah, the Mighty and Majestic. But I do not find that which I can carry them upon, and they do not find the capacity to follow me, and their souls do not feel at ease to remain behind after me."