Musnad Ahmad 26:831

Ya'qub narrated to us, he said: My father narrated to me, from Ibn Ishaq, he said: 'Abd al-Rahman ibn al-Aswad ibn Yazid al-Nakha'i narrated to me, from his father, from 'Abdullah ibn Mas'ud, he said: The Messenger of Allah (may Allah bless him and grant him peace) taught me the Tashahhud in the middle of the prayer and at the end of it. We used to memorize from 'Abdullah when he informed us that the Messenger of Allah (may Allah bless him and grant him peace) taught it to him. He said: When he sat in the middle of the prayer and at the end of it upon his left hip, he would say: "The greetings are for Allah, and the prayers and the good things. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us and upon the righteous servants of Allah. I bear witness that there is no god but Allah, and I bear witness that Muhammad is His servant and His messenger." He said: Then if he was in the middle of the prayer, he would rise when he finished his Tashahhud. And if he was at the end of it, he would supplicate after his Tashahhud with whatever Allah willed for him to supplicate, then he would perform the Taslim.