Yahya related to me from Malik from Ibn Shihab that Abu Umama ibnSahl ibn Hunayf said, "Zayd ibn Thabit entered the mosque and foundthe people in ruku, so he went into ruku and then moved slowly forwarduntil he reached the row."
USC-MSA web (English) reference: Book 9, Hadith 67
Arabic reference: Book 9, Hadith 398