It is narrated from Ibn Shihab (may Allah have mercy on him): Abu Umamah bin Sahl bin Hunayf (may Allah be pleased with him) informed me that he saw Zayd bin Thabit (may Allah be pleased with him) enter the mosque while the imam was bowing. He walked on until it became possible for him to reach the row while bowing, then he said the takbir and went into bowing, then, while bowing, he walked slowly until he joined the row.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2586