Maliks Muwatta Book 9, Hadith Number 67.
Section : What to Do when Entering while the Imam is in Ruku.
Yahya related to me from Malik from Ibn Shihab that Abu Umama ibn Sahl ibn Hunayf said, “Zayd ibn Thabit entered the mosque and found the people in ruku, so he went into ruku and then moved slowly forward until he reached the row.”