Musnad Ahmad 788:241

Khalaf ibn al-Walid narrated to us, saying: Khalid—meaning al-Tahhan—narrated to us, from Mutarrif, from Amir, from Abu Burdah, from Abu Musa, that the Messenger of Allah (peace and blessings of Allah be upon him) said regarding the one who emancipates a slave girl and then marries her: "He has two rewards."