It is narrated from Umar bin Abd al-Aziz (may Allah have mercy on him) that when a person finds the people bowing, he should say two takbirs, one to begin the prayer and the other for bowing; when he has done so, he has caught the rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2591