1. The Prophet Muhammad (SAWA) has ordered us to do so as it clear in the Hadeeth : When you say Laa ELAAHA Illallah, Muhammad Rasoolullah, say : :Ali Ameerul Mo'mineen. (Kitab Al-Ehtijaj, page 158).
2. Hadeeth of the Prophet (SAWA): When I was elevated to the Sky, I saw writing on the Throne العرش : LAA ELAAHA ILLALLAH, MUHAMMAD RASOOLULLAH, AYYADTOHO BI ALI لا إله إلا الله محمد رسول الله، أيّدتُهُ بِعليّ
This Hadeeth was narrated not only in Shia books but also in many Sunni books e.g. Al-Manaaqib by Ibn Al-Maghazili, page 93, Al-Shaafi by Qadhi E'yaadh 138, Al-Durr Al-Manthour by Al-Soyooti 4:153, Al-Khasaa'is Al-Khubra by Al-Soyooti 1:7.
3. Hadeeth narrated by Abdullah Ibn Mas'ood from the Prophet Muhammad (SAWA): An angle came to me saying : O Muhammad, ask the previous prophets and messengers who were been sent before you; what was their message? I asked him: What was their message? He said: They were been sent on following your message and following Ali Ibn Abi Talib. ( Ma'rifat 'Oloom Al-Hadeeth by the wellknown Sunni imam al-Haakim Al-Nisaburi , p. 96).
4. Hadeeth of the Prophet to Ali: I asked Allah to keep your name with my name whenever I will be mentioned, and He granted me that.( Kanzul Ommal 13:113).
5. After the Prophetic announcement of Al-Ghadeer ( For whom I am the master, Ali is his master), Athaan (Azaan) was declared with Ashhadu Anna Aliyyan Ameerul Mo'mineen) (Al-Salaafah Fi Tareekh Al-Khilafah by the Egyptian Sunni Alim Al-Maraaghi, page 36 and 38.