This is a well proven hadith from the Sunni sources. Imam Tirmidhi also wrote this hadith:

"Ali is from and I am from Ali, He is the WALI (Master) of all believers."

We can understand that the Holy Prophet(pbuh) said about Imam Ali that  Ali  is from him because the Imam is the cousin of the Holy Prophet and son-in-law who was raised in the hands of Prophet but when the Prophet said “I am from Ali” this is not a light thing. It is a huge confirmation about the status of Imam Ali.

It clearly means that after the Holy Prophet Imam Ali is the true picture of Prophet Mohammad (pbuh). 

English Translation:

Bara ibn Aazib reported that the Prophet (SAW) said to Ali ibn Abu Talib,

"You are from me, I am from you."

Imam Tirmidhi quoted that this is Hassan Sahih Hadith.