Imam Malik (may Allah have mercy on him) related that it had reached him that Abdullah bin Umar and Zayd bin Thabit (may Allah be pleased with them both) both used to say that whoever catches the bowing before the imam raises his head has caught the rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2582