It is narrated from Sayyiduna Uthman bin Abi al-As (may Allah be pleased with him) that when the delegation of Thaqif came to the Prophet (peace and blessings be upon him), he (peace and blessings be upon him) lodged them in the mosque so that it would soften their hearts. They stipulated to the Prophet (peace and blessings be upon him) that they would not be mustered (for the army), would not have the tithe ('ushr) collected from them, would not be required to bow (that is, to perform ruku' in prayer), and that no one other than themselves would be appointed over them. Upon this the Prophet (peace and blessings be upon him) said: "There is no good in a religion that has no bowing (ruku', i.e. prayer) in it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8340