Musnad Ahmad 1012:54

Mu'awiyah narrated to us, Abu Ishaq narrated to us, from 'Abd al-Rahman ibn 'Ayyash, from Sulayman ibn Musa, from Makhul, from Abu Umamah, from 'Ubadah ibn al-Samit, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "You must engage in Jihad in the way of Allah, for it is a gate from the gates of Paradise, through which Allah removes worry and grief."