Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: The Prophet (peace and blessings be upon him) said: "Whoever recites Surah al-Fatihah and Surah al-Ikhlas, it is as though he has recited one third of the Qur'an."
Al-Tabarani recorded it in al-Mu'jam al-Awsat. Its chain contains Sulayman bin Ahmad al-Wasiti, who is matruk (abandoned).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب التفسير / 10815