It is narrated from Abd al-Rahman al-Asamm: I asked Anas bin Malik (may Allah be pleased with him) about the takbir in the prayer, and he said: He would say the takbir when he bowed and when he prostrated, and when he raised his head from the prostration, and when he prostrated again, or when he rose after two rak'ahs, he would also say the takbir. Khatim asked him: From whom have you memorised this? He said: From the Prophet (peace and blessings be upon him), and from Abu Bakr al-Siddiq and Umar al-Faruq (may Allah be pleased with them both). Khatim said to him: And from Uthman (may Allah be pleased with him) too? He said: Yes! From Uthman (may Allah be pleased with him) too. The word is Khatim; some have also said Hatim.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2498