Musnad Ahmad 299:6

Yunus narrated to us, he said: Hammad—meaning Ibn Zayd—narrated to us, from Muhammad ibn Ishaq, from Sa'id ibn Abi Hind, from Mutarrif, who said: I entered upon Uthman ibn Abi al-As, and he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Fasting is a shield, like the shield of one of you in battle." And the last thing the Messenger of Allah, peace and blessings of Allah be upon him, enjoined upon me when he sent me to Ta'if was: "O Uthman, be brief in the prayer, for among the people are the elderly and those with needs."