Sahih Bukhari, Hadith: Q784, (115) Chapter. Itmam At-Takbir (i.e., to end the number of Takbir or to say the Takbir perfectly) on bowing. [See Fath Al-Bari ].

صحيح البخاري

Sahih Bukhari

كتاب الأذان (صفة الصلوة)

The Book of Adhan (Sufa-tus-Salat)

باب إتمام التكبير فى الركوع:

(115) Chapter. Itmam At-Takbir (i.e., to end the number of Takbir or to say the Takbir perfectly) on bowing. [See Fath Al-Bari ].