It is narrated from a Companion (may Allah be pleased with him) of the Messenger of Allah (peace and blessings be upon him) that he said: I heard this noble saying from the blessed tongue of the Messenger of Allah (peace and blessings be upon him) more than twenty times; he was saying: "How excellent and fine are the two surahs that I recited in the two rak'ahs (of the Sunnah): (one) 'Say: He is Allah, the One; Allah, the Eternal Refuge' [Surah al-Ikhlas: 1-2] and (the other) 'Say: O disbelievers' [Surah al-Kafirun: 1]."
Imam Musaddad bin Musarhad recorded this hadith in his Musnad. Although one narrator in its chain is unnamed, the vagueness about the identity of a Companion does not harm the authority of a report, because all the noble Companions (may Allah be pleased with them) are upright and reliable.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب التفسير / 3785