Imam Al-Nawawi’s Riyad-us-Saliheen
Sunnah of the ‘Isha’ Prayer
[Ibn `Umar (May Allah be pleased with them) has narrated that he performed two Rak`ah after the Fard prayer of `Isha’ with the Prophet (sallallaahu alayhi wa sallam). `Abdullah bin Maghaffal has narrated that the Messenger of Allah (sallallaahu alayhi wa sallam) has said, “There is a Salat between every Takbir and Adhan.”
This proves that apart from the four Rak`ah Fard, there are also two Rak`ah Sunnah of the `Isha’ prayer). See Ahadith No. 1098 and 1099.