Musnad Ahmad 1012:91

Sulayman ibn Dawud al-Hashimi narrated to us, Isma'il informed us, 'Amr informed us, from al-Muttalib, from 'Ubadah ibn al-Samit, that the Prophet, peace and blessings of Allah be upon him, said: "Guarantee for me six things from yourselves, and I will guarantee Paradise for you: be truthful when you speak, fulfill when you promise, render back when you are entrusted, guard your private parts, lower your gaze, and restrain your hands."