(a) Nafi' (may Allah have mercy on him) said: When Sayyiduna Ibn Umar (may Allah be pleased with them both) found that the imam had already prayed part of the prayer, he would pray with the imam whatever he caught. If the imam stood, he stood, and if the imam sat, he sat, until the imam completed his prayer. He would not differ from the imam but would follow him.
(b) Nafi' (may Allah have mercy on him) said that Sayyiduna Ibn Umar (may Allah be pleased with them both) used to say: When you miss the bowing, you have missed the rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3620