Of the Adhkar (invocations and Remembrances said at certain times on a regular basis) that are connected with Salah is Adhkar At-Tashah-hud (a recitation in the sitting position in the second/ last unit of Prayer). It was authentically reported from the Prophet (peace be upon him) several Hadiths that contain similar forms of Tashah-hud which are all permissible. Of which is the Hadith that was reported in Sahih Muslim from `Abdullah ibn `Abbas (may Allah be pleased with them) who said: "The Messenger of Allah (peace be upon him) used to teach us Tashah-hud just as he used to teach us a Surah of the Qur'an, and he would say: At-Tahiyyatu Lillah Was-Salawatu Wat-Tayyibat, As-Salamu `Alayka Ayyuha An-Nabiy Warahmatullahi Wabarakatuh As-Salamu `Alayna Wa`la `Ibadillah As-Salihin [All good and blessed greetings are due to Allah. Peace, mercy, and blessings of Allah be upon you O Prophet. Peace be upon us and upon Allah's upright servants. I bear witness that there is none worthy of worship (in truth) but Allah, and I bear witness that Muhammad is the Messenger of Allah]." [Sahih Muslim: No. 403].
It was authentically reported in Sahih Al Bukhari and Sahih Muslim on the authority of `Abdullah ibn Mas`ud (may Allah be pleased) with him: "Whenever we offered Salah behind the Prophet (peace be upon him), we used to recite (in sitting) 'Peace be on Gabriel, Michael, peace be on so and so. Once Allah's Messenger (peace be upon him) looked back at us and said: Allah Himself is As-Salam (Peace), so when anyone of you offers Salah, he should say: “At-Tahiyyatu Lillahi Was-Salawatu Wat-Tayyibat As-Salamu `Alayka Ayyuha An-Nabiyyu Warahmatu Allahy Wabarakatuh As-Salam `Alayna Wa`Ala `Ibadi Allahy As-Salihin.” [Salutations are for Allah. All acts of worship and good deeds are for Him. Peace, mercy, and blessings of Allah be upon you O Prophet]. Peace be upon us and upon Allah's upright servants. If you say that, it will be for every pious servant in the heaven and the earth. “Ashhadu Alla Ilaha Illa Allah Wa Ashhadu Anna Muhammadan `Abduhu Warasuluh.” (I testify that none has the right to be worshipped but Allah and I also testify that Muhammad is His slave and His Messenger)." [Sahih Al Bukhari: No. 831, and Sahih Muslim: No. 402].
Many Hadiths were reported in this regard.
The most perfect form is the form that was reported in the Hadith of Ibn Mas`ud because it is the most perfect, as Ibn Al Qayyim (may Allah bestow mercy on his soul) said: The Tashah-hud of Ibn Mas`ud included separate phrases, whereas the Tashah-hud of Ibn `Abbas included one phrase. [The book of Salah: P. 211]. Hence, each phrase reported in the Hadith of Ibn Mas`ud is considered a separate praise because of the conjunction "And" in the Prophet's saying: "At-Tahiyyatu Lillahi was-Salawatu wat-Tayyibat," however if it is omitted, the phrase will be a description to the previous sentence. So, the multiplication of praise in the Hadith of Ibn Mas`ud is explicit, therefore it is the most perfect. Moreover, it is known to all scholars and the most correct chain of transmission in this regard. At-Tirmidhy (may Allah bestow mercy on his soul) said: The Hadith of Ibn Mas`ud was transmitted from many ways, and it is the most correct Hadith reported from the Messenger of Allah (peace be upon him) regarding Tashah-hud. Furthermore, it is the most authentic Hadith which most scholars have adopted from the Companions and the Followers. [Sunan At-Tirmidhy: 2/82]. Nevertheless, saying this form of Tashah-hud or others is permissible and correct. As for the Prophet's saying: "At-Tahiyyat," it is the plural form of Tahiyyah, and the meaning is glorifications by all their forms: Bowing down, prostration, humbleness, and submission. All these are for Allah alone without associating partners with Him in worship, and they all due to Him. As for the Prophet's saying: "Was-Salawat," it was said that the meaning is the Salah which has bowing down and prostration. It was said also that the meaning is invocation because Salah in linguistics is invocation. All these actions are due to Allah, so it is not proper to direct any act of worship to other than Allah. As for the Prophet's saying: "Wat-Tayyibat," is the plural form of Tayyibah, and the meaning is good words and actions which are meant for Allah and none of which is given to anyone but Him because people draw nearer to Him alone by all good words and actions. As for the Prophet's saying: "As-Salamu `Alayka Ayyuhan-Naby warahmatullahi wabarakatuh," this is an invocation to the Prophet (peace be upon him) with peace, mercy, and blessing. As for the Prophet's saying: "As-Salamu `Alayna wa`ala `Ibadillahis-Salihin," it includes invocation for oneself and for all the believers with safety from every defect, shortage, evil because it is one of the short inclusive words of the Prophet (peace be upon him). Some scholars said: The Prophet (peace be upon him) instructed the nation to single out Allah with Adhkar because of His Honor and Great due rights on them, then he instructed them to mention themselves first because it is the most important then commanded them to generalize the invocation to include peace upon the pious servants of Allah to inform the people that invoking Allah for the believers should be inclusive. [Fathul-Bary of Ibn Hajar (2/313) reported from Al Baydawy].