Ata bin Abi Rabah (may Allah have mercy on him) says that he heard Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with him) while he was on the pulpit saying to the people: "When one of you enters the mosque and the people are bowing, let him bow as soon as he enters, then walk slowly in the state of bowing until he joins the row, for that is the Sunnah." Ata (may Allah have mercy on him) says: I saw him doing that himself.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5218