Kurayb, the freed slave of Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), narrates that Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) and some people from among the Companions of the Prophet (peace and blessings be upon him) sent him to Sayyidah Umm Salamah (may Allah be pleased with her) to ask her about the days on which he (peace and blessings be upon him) most often fasted. She said: Saturday and Sunday. Then I returned to them and informed them, and it was as though they found it strange, so they all went to her (and said): We sent him to you for this matter, and he says that you (may Allah be pleased with her) said this. She said: He has spoken the truth; indeed the Messenger of Allah (peace and blessings be upon him) most often fasted on Saturday and Sunday, and he used to say: "These were days of festival for the polytheists, and I want to differ from them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8497