Musnad Ahmad 1156:278

'Attab ibn Ziyad narrated to us, he said: 'Abdullah—meaning Ibn al-Mubarak—narrated to us, he said: 'Abdullah ibn Muhammad ibn 'Umar ibn 'Ali informed me, he said: My father narrated to me, from Kurayb, that he heard Umm Salamah saying: The Messenger of Allah (peace and blessings of Allah be upon him) used to fast on Saturday and Sunday more than he would fast on other days. And he would say: "They are two days of festival for the polytheists, and I love to oppose them."