Musnad Ahmad 8:39

Waki’ narrated to us, Usamah bin Zaid narrated to us, from Muhammad bin Abd al-Rahman bin Abi Labibah, from Sa’d bin Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best of remembrance is the hidden [one], and the best of provision is that which suffices."