Musnad Ahmad 28:295

Muhammad ibn Ja‘far narrated to us, Sa‘id narrated to us, from Qatadah, from Sa‘id ibn al-Musayyib, from ‘Abdullah ibn ‘Amr, that the Messenger of Allah (peace and blessings of Allah be upon him) entered upon Juwayriyah bint al-Harith while she was fasting on a Friday. He said to her, "Did you fast yesterday?" She said, "No." He said, "Do you intend to fast tomorrow?" She said, "No." He said, "Then break your fast." Sa‘id said: Matar agreed with me regarding it, from Sa‘id ibn al-Musayyib.