Musnad Ahmad 30:1012

And the Messenger of Allah, may Allah bless him and grant him peace, said: "By the One in Whose hand is the soul of Muhammad, were it not that I would be burdensome to the believers, I would not remain behind any expedition fighting in the way of Allah. But I do not find the means to carry them, and they do not find the means to follow me, and their souls do not find it easy to remain behind after me."