Musnad Ahmad 32:546

Abdullah ibn al-Walid narrated to us, Sufyan narrated to us, he said: Salamah ibn Wardan narrated to me, he said: I heard Anas ibn Malik saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Say, 'O disbelievers' (Surah Al-Kafirun) is a quarter of the Quran. And 'When the earth is shaken' (Surah Az-Zalzalah) is a quarter of the Quran. And 'When the victory of Allah has come' (Surah An-Nasr) is a quarter of the Quran."