In books it mentions that after the Fardh and Sunnah for the above prayers you can optionally pray 2 Rak'at Nafl. Which Hadith says this please?
You may find evidences for that in this search on sunnah.com
So it is mentioned in almost all known hadith compilations on the authority of at least 5 sahabah. For Example in Sahih al-Bukhari:
I remember ten Rakat of Nawafil from the Prophet, two Rakat before the Zuhr prayer and two after it; two Rakat after Maghrib prayer in his house, and two Rakat after 'Isha' prayer in his house, and two Rakat before the Fajr prayer and at that time nobody would enter the house of the Prophet.
Hafsa told me that the Prophet (ﷺ) used to offer two Rakat after the call maker had made the Adhan and the day had dawned.