The one who catches up with a rak`ah of the prayer, he has caught up with that prayer
وَحَدَّثَنَاهُ عَبْدُ الأَعْلَى بْنُ حَمَّادٍ، حَدَّثَنَا مُعْتَمِرٌ، قَالَ سَمِعْتُ مَعْمَرًا، بِهَذَا الإِسْنَادِ .
This hadith has been reported by Ma'mar with another chain of transmitters.