1. Allaah, the Most High says:
"None dispute about the signs of Allaah, except those who disbelieve." (Soorah Ghaafir 40:4)
At-Tirmidhee reports a hasan hadeeth from Abu Umaamah, who said: "Allaah's Messenger (saws) said, "A people never went astray after being upon guidance except through disputation." Then Allaah's Messenger (saws) recited this Ayah :
"This they set forth to you, only by way of argument. Nay, but they are a contentious people." (Soorah az-Zukhruf 43:58)
Al-Aajurree reports in ash-Sharee'ah (p.57) that a man came to al-Hasan (al-Basree) and said, "O Abu Sa'eed! Let me debate with you about the Religion." Al-Hasan replied, "As for me, I know my religion, If you have lost your Religion then go and look for it."
'Umar ibn 'Abdul-'Azeez, rahimahullaah, said, "He who allows his Religion to be open to disputing will frequently change over." See Jaami' Bayaanil 'Ilm (2/113)