Can one perform the two rak'ahs of the 'Isha supererogatory prayer while sitting?

The Sunnah of 'Isha' consists of two confirmed rak'ahs which the Prophet, peace and blessings be upon him, regularly observed. The proof for this is the hadith of Ibn 'Umar, may Allah be pleased with them both. There is no evidence that the Sunnah of 'Isha' can only be prayed while sitting. However, if a person prays these two rak'ahs while sitting, or prays the Shafa' (even-numbered rak'ahs before Witr) while sitting, it is permissible for him. All supererogatory (nafl) prayers are valid when performed sitting, but the reward for one who prays sitting is half the reward of one who prays standing.