It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "O son of Adam, guarantee Me two rak'ahs at the beginning of the day, and I will suffice you for the rest of it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13443