Musnad Ahmad 937:189

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Ayyub, from Abu al-'Aliyah al-Barra', from 'Abdullah ibn al-Samit, from Abu Dharr, from the Prophet, may Allah bless him and grant him peace, that he said: "O Abu Dharr, how will you be when you remain among a people who delay the prayer beyond its time?" He said: He said to me: "Perform the prayer at its time. Then, if you reach them while they have not prayed, pray with them, and do not say: 'I have already prayed, so I will not pray.'"