It is narrated from Abu Bakr bin Abd al-Rahman that Sayyiduna Abu Bakr al-Siddiq and Sayyiduna Zayd bin Thabit (may Allah be pleased with them both) entered the mosque while the imam was bowing. They both bowed at once, then, while still in the state of bowing, they walked until they joined the row.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2585