Ibn e Ishak narrated that:
A scholar has stated that the Messenger of Allah used to hide from Abu Talib and other relatives and go to the outskirts of Medina along with Ali, where both of them would pray together till they returned back in the evening.
One day, Abu Talib happened to see them both while they were offering congregational prayers. Abu Talib said to the Messenger of Allah, "O my nephew, what is this religion that you are following?" The Messenger of Allah said, "Uncle, this is the religion of Allah , the angels, the prophets, and our father Ebrahim."
Some people say that Abu Talib also said to his son Ali, "My son, what is this religion which you believe in?" Imam Ali (peace be upon him) replied, father I believe in Allah and His Messenger and offer prayers along with him.
To this Abutalib said, do bear in mind that the Prophet has not called you to anything but goodness, so do associate with him.
Ref: Ibn Hisham Al Badaya Vol. 3 Page 56; Tabari, Tarikh volume 2 page 214