This must be as a result of two things - either the Holy Prophet (s.
This must be as a result of two things - either the Holy Prophet (s.a.w.) had ordered him to do so or else this was Abu Dharr's understanding of the statements of the Holy prophet (s.a.w.) and his (s.a.w.) stance. Anyhow, the Holy Prophet (s.a.w.) has made his (s.a.w.) move. It is agreed by the scholars and the jurists that the words of the Holy Prophet (s.a.w.), his (s.a.w.) actions and his (s.a.w.) sermon is proof of law. Okay, so this Hadeeth comes from a non-Shiite source.
Sometimes it happens that even the adversaries concur with your position and then you can point to them and say that 'See! Even they agree with us!' Incidentally, there is also another Hadeeth from non-Shiite sources from Abu Laith Al Harwi where it is stated that the Third Testimony did form part of the Adhaan during the time of the Holy Prophet (s.a.w.), but soon after was stopped by the ruling Caliphs who claimed it to have been abrogated.
The main Hadeeth from Shiite sources for the third testimony is the one below: BIHAR UL ANWAAR - VOL 27 CH1 H1 It has been related that Qasim Bin Muwaiya said to the Holy Imam Ja'far Al Sadiq (a.s): 'The people of the Sunnah relate a Hadeeth about Me'raaj that when the Holy Prophet (s.a.w.) was taken on Me'raaj he (s.a.w.) saw written upon the Throne - There is no God but Allah, Muhammad is the Messenger of Allah and Abu Bakr As Siddiq'. He (a.s.) said: 'Glory be to Allah (s.w.t.)!
They have changed everything, even this?' He said: 'Yes!' The Holy Imam (a.s.) continued: 'When Allah (s.w.t.) the High created the Throne, He (s.w.t.) wrote upon it - There is no God but Allah, Muhammad is the Messenger of Allah and Ali is the Commander of the believers. And when He (s.w.t.) the High Created the Chair, He (s.w.t.) wrote upon it - There is no God but Allah, Muhammad is the Messenger of Allah and Ali is the Commander of the believers.
And when He (s.w.t.) Created the Tablet, He (s.w.t.) wrote on it - He (s.w.t.) wrote upon it - There is no God but Allah, Muhammad is the Messenger of Allah and Ali is the Commander of the believers. And when He (s.w.t.) the High Created Israfeel (the angel) he (s.w.t.) wrote upon his forehead - There is no God but Allah, Muhammad is the Messenger of Allah and Ali is the Commander of the believers.
And when He (s.w.t.) Created the heavens he (s.w.t.) wrote on its fringes - There is no God but Allah, Muhammad is the Messenger of Allah and Ali is the Commander of the believers.