Ash-Shaykh Bin Baaz, rahimahullah, said:
نعم، لا بأس بالصلاة بعد الوتر، إذا أوتر الإنسان في أول الليل أو في وسط الليل ثم يسر الله له القيام في آخر الليل فإنه يشرع له أن يصلي ما قسم الله له: ركعتين أو أكثر من ذلك، ويكفيه الوتر الأول لا حاجة إلى وتر ثانٍ،… الصلاة بعد الوتر جائزة، وإنما الأفضل أن يكون الوتر آخر الشيء
“Yes, there is no harm in offering [supererogatory] prayers after the witr prayer. If a person were to pray witr in the first part of the night or in the second part of the night, then it will be legislated for him to pray to what Allah has decreed for him whether that be two rak’ahs or more and the first witr is sufficient for him since there is no need for a second witr…To offer salah after witr is permissible and it is only preferable that the witr prayer be the last prayer… ”
The Lajnah ad-Da’imah stated:
It is also narrated by Ahmad on the authority of Um Salamah (may Allah be pleased with her) that the Prophet (peace be upon him) used to offer two Rak`ahs after praying the Witr while he was sitting. The Messenger of Allah (peace be upon him) did that to show the permissibility of offering Salah after performing Witr Prayer, and that such Salah does not come in contradiction with the Hadiths related to Witr Prayer, and that offering it does not forbid offering supererogatory prayers at a later time at night.
AbdulFattaah Bin Uthman