Musnad Ahmad 1017:86

Ahmad ibn Abd al-Malik narrated to us, Musa ibn A'yan narrated to us, from Layth, from Alqamah ibn Marthad, from Sulayman ibn Buraydah, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "They have what they embraced Islam upon of their lands, their slaves, and their livestock, and there is nothing upon them regarding it except the Sadaqah (Zakat)."