Musnad Ahmad 30:393

Yazid narrated to us, Hisham informed us, from Yahya, from Abu Ja'far, that he heard Abu Hurairah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best of deeds in the sight of Allah is: faith in which there is no doubt, and a military expedition in which there is no misappropriation of spoils, and an accepted Hajj." And Abu Hurairah said: "An accepted Hajj expiates the sins of that year."