سنن النسائي

It was narrated that 'Abdur-Rahman bin Al-Asamm said: "Anas bin Malik was asked about the takbir in the prayer. He said: "The takbir should be said when bowing, when prostrating, when raising one's head from prostration and when standing up following the first two rak'ahs.' Hutaim said: 'From whom did you learn this?' He said: 'From the Prophet (ﷺ), Abu Bakr and 'Umar, may Allah (SWT) be pleased with them.' Then he fell silent and Hutaim said to him: 'And 'Uthman?' He said: 'And 'Uthman.'"