The tashahhud

It is narrated from Abu Ishaq that he said: I met al-Aswad bin Yazid and said to him: Indeed, Abu al-Ahwas adds the word "al-mubarakat" in the narration of the tashahhud from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him). At this al-Aswad said: Go to him and stop him from this addition, and tell him that Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) taught Alqamah the words of the tashahhud and counted them on the fingers of his hand (to fix their number and wording firmly).
This hadith is sahih (authentic).

It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he said: "The Messenger of Allah (peace and blessings be upon him) used to teach us the tashahhud (al-tahiyyat) with as much attention and regularity as a teacher teaches children to write."
In this same hadith narrated by Musaddad, Abd al-Wahid also transmitted, by way of Abd al-Rahman bin Ishaq, the addition that he (peace and blessings be upon him) used to give this teaching seated "on the pulpit."
It is also narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he said: "Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) used to teach us the tashahhud on the pulpit just as children are taught in the maktab (school)."

It is narrated from Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "I have been given the eloquent openings of speech, its most comprehensive and meaningful words, and its finest conclusions." We said: "Teach us too some of the knowledge that Allah Almighty has given you." So he (peace and blessings be upon him) taught us the tashahhud.
The chain of this hadith contains a narrator named Abd al-Rahman bin Ishaq al-Kufi, who is da'if (weak); however, other authentic hadith are narrated concerning the teaching of the tashahhud.

It is narrated from Khusayf (may Allah have mercy on him) that he said: I saw the Messenger of Allah (peace and blessings be upon him) in a dream and said: "O Messenger of Allah! A disagreement has arisen among us about the tashahhud (reciting al-tahiyyat in the prayer): so-and-so says it this way, and so-and-so says it that way, while Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) recites it this way." The Prophet (peace and blessings be upon him) said: "How excellent and fine a tashahhud is the tashahhud of Ibn Mas'ud (may Allah be pleased with him)!"
And al-Tirmidhi narrated it in abridged form.

It is narrated from Sayyidah Aishah (may Allah be pleased with her): "Indeed, the Messenger of Allah (peace and blessings be upon him) did not add any words to the tashahhud in (the first sitting after) the (opening) two units (of the prayer)."
Abu Ya'la recorded this hadith in his Musnad, and its chain is sahih (authentic) on the condition of Muslim.