Q: Since when was the testimony of AmirulMomineen Ali (as) added in the Adhan and Iqamat?
«اشهد ان علیا ولی الله »
A: This slogan, testimony was practiced during the life of the Holy Prophet (sa).
Some of the noble companions of the Apostle (sa), like Salman & Abazar (as) were persistent in mentioning it in their Adhan & Iqamat and the Prophet (sa) too allowed this noble act.
The fatwa of Shia ulama states it to be a recommended/ mustahabb practice.