During the famous battle of Qadisiyyah, the Persian general Rustam sent word to the Muslim commander Sa‘d ibn Abi Waqqas 
to send an envoy for negotiations. The latter chose his fellow Sahabi Rib‘i bin ‘Amir as the envoy.
When Rib‘i arrived at Rustam’s court, he was asked by the general through the interpreter: “What is your purpose in waging war against us?”
Rib‘i bin ‘Amir replied:
“Allah has sent us forward so that we may liberate, whomsoever He wills, from following men [and lead them] to the obedience of Allah, and pull them out of their narrow world into the broader one, and from under the suppression of [various] religions into the justice of Islam…”
[Ibn Kathir, al-Bidāyah wa al-Nihāyah, Cairo, Sa‘adah Press, vol. 7, p. 39. Cited by Siddiqi, M N, ‘Tawḥīd: The Concept and the Process’ In Islamic Perspectives, Islamic Foundation, Leicester, 1979, pp. 20-21]
The Caliphate 