Musnad Ahmad 962:98

Yahya ibn Ishaq al-Saylahini narrated to us, Ibn Lahi'ah narrated to us, from Sulayman ibn Abd al-Rahman, from al-Qasim, from Abu Umamah, who said: I was beneath the mount of the Messenger of Allah, peace and blessings of Allah be upon him, on the day of the Conquest, and he said a good, beautiful statement. Among what he said was: "Whoever embraces Islam from the People of the Two Books, he has his reward twice, and he has what we have and upon him is what is upon us. And whoever embraces Islam from the polytheists, he has his reward, and he has what we have and upon him is what is upon us."