Musnad Abu Ya'la al-Mawsili 46:2

Abu Bakr narrated to us, Waki' narrated to us, from Sufyan, from Salamah, from al-Qasim ibn Mukhaymirah, from Abu 'Ammar, from Qays ibn Sa'd, who said: The Messenger of Allah (may Allah bless him and grant him peace) commanded us to pay Zakat al-Fitr before the Zakat was revealed. When the Zakat was revealed, he neither commanded us nor forbade us, and we continue to do it.