Musnad Ahmad 1105:4

Waki' narrated to us, Sufyan narrated to us, from Salamah ibn Kuhayl, from al-Qasim ibn Mukhaymirah, from Abu 'Ammar al-Hamdani, from Qays ibn Sa'd, who said: "The Messenger of Allah, may Allah bless him and grant him peace, commanded us with Sadaqat al-Fitr before Zakat was revealed. When Zakat was revealed, he did not command us [to do it], nor did he forbid us [from doing it], and we were doing it."