Abu Dawud Book 18, Hadith Number 3587.

Narated By Abu Hurairah : The Prophet (PBUH) said: Conciliation between Muslims is permissible. The narrator Ahmad added in his version: “except the conciliation which makes lawful unlawful and unlawful lawful.” Sulayman ibn Dawud added: The Apostle of Allah (PBUH) said: Muslims are on (i.e. stick to) their conditions.