Musnad Ahmad 30:381

Yazid narrated to us, Muhammad ibn Ishaq informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, who said: Abu al-Qasim (peace and blessings of Allah be upon him) said: "By the One in Whose hand is the soul of Muhammad, if you knew what I know, you would have wept much and laughed little."