From Sahih Muslim (Hadith 1886):
Abu Darda' reported Allah's Apostle (ﷺ) as saying:Is any one of you incapable of reciting a third of the Qur'an in a night? They (the Companions) asked: How could one recite a third of the Qur'an (in a night)? Upon this he (the Holy Prophet) said: "'Say: He is Allah, One' (Qur'an. cxii) is equivalent to a third of the Qur'an
Reference: Muslim 1886