(a) Sayyidah Aishah (may Allah be pleased with her), describing the prayer of the Messenger of Allah (peace and blessings be upon him), said that the Messenger of Allah (peace and blessings be upon him) would recite the tashahhud between every two rak'ahs.
(b) In the narration of Husayn bin Dhakwan al-Mu'allim in Sahih Muslim, she said: He (peace and blessings be upon him) used to say: "In every two rak'ahs there is the tashahhud."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2797
Ali bin Yahya bin Khallad bin Rafi' narrates from his father, and he from his uncle Rifa'ah bin Rafi' (may Allah be pleased with him), that the Prophet (peace and blessings be upon him) said: "When you stand for your prayer, say 'Allahu Akbar' (Allah is the Greatest), then recite whatever you know of the Qur'an." It also contains: "When you sit in the middle of the prayer, sit calmly and lay your left thigh flat, then recite the tashahhud; then when you stand (for the third rak'ah), do the same, until you finish your prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2798