(a) Sayyiduna Umar bin al-Khattab and Sayyiduna Abu al-Darda' (may Allah be pleased with them both) said: Whatever you catch of the last part of the imam's prayer, count it as the first part of your own prayer.
(b) Al-Walid (may Allah have mercy on him) said: I mentioned this to al-Awza'i (may Allah have mercy on him) and Sa'id bin Abd al-Aziz (may Allah have mercy on him), and they said: Whatever you catch of the imam's prayer is the first part of your prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3632