Musannaf Ibn Abi Shayba 3:4737

Abu Bakr narrated to us, he said: Isma'il ibn 'Ayyash narrated to us, from Sa'id ibn Abi 'Abd al-Rahman, that 'Umar ibn al-Khattab and Abu al-Darda' used to say: "Whatever you catch of the Imam's prayer, make it the beginning of your prayer."