Musnad Ahmad 1012:61

Waki’ narrated to us, Sufyan narrated to us, from ‘Abd al-Rahman ibn al-Harith ibn ‘Ayyash ibn Abi Rabi’ah, from Sulayman ibn Musa, from Makhul, from Abu Salam al-A’raj, from Abu Umamah, from ‘Ubadah ibn al-Samit, that the Prophet, peace and blessings of Allah be upon him, said: "He gave as a voluntary gift (nafal) in the initial advance one-fourth, and in the return one-third."