Musnad Ahmad 30:1845

Muhammad ibn Ja'far narrated to us, he said: 'Awf narrated to us, from al-Hasan, he said: It reached me that the Messenger of Allah, peace and blessings of Allah be upon him, said: "When the call to prayer is made, do not come to it running, but walk with tranquility upon you. So whatever you catch, pray it, and whatever you missed, complete it."