Musnad Ahmad 937:190

Abu ‘Amir narrated to us, Shu‘bah narrated to us, from Budayl ibn Maysarah, who said: I heard Abu al-‘Aliyah al-Barra’, from ‘Abdullah ibn al-Samit, from Abu Dharr, that the Prophet, peace and blessings of Allah be upon him, struck his thigh and said to him: "How will you be when you remain among a people who delay the prayer?" Then he said: "Perform the prayer at its time, then rise. If you are in the mosque when the prayer is established, then pray with them."