Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.011A.
Section : Separating The Obligatory Prayer From The Supererogatory.
It is preferred to make a separation between the fard and nawafil prayers after one finishes the fard prayer.
One of the companions of the Prophet sallallahu alehi wasallam reports that the Prophet prayed the afternoon prayer and right afterward a man stood up to pray. ‘Umar saw him and told him: “Sit, the People of the Book were destroyed because they did not differentiate between their prayers.” The Prophet said: “Well said, Ibn al-Khattab [i.e. ‘Umar].” This is related by Ahmad with a sahih chain.