All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The Du’aa' mentioned by the questioning brother, based upon the saying of the Prophet "He who recites ten times before getting up (from the place of worship) and bending his steps back from the Maghrib and the Fajr Prayers (There is no god but Allah, the One, having no partner with Him, to Him belongs sovereignty and to Him all praise is due; in His hand is the good; He gives life and causes death; and He is Omnipotent over everything) ten times, for him will be recorded for every recital ten good deeds and ten of his sins are wiped out…" The words mentioned in the above Hadeeth are: “He who recites ten times before getting up and bending his steps back from the Maghrib and the Fajr Prayers…” These words are only narrated by one narrator Shahr ibn Hawshab who is a weak narrator according to the scholars of Hadeeth, consequently these words are not authentic as ruled by the scholars of Hadeeth. The Hadeeth without the particulars mentioned words is correct. Therefore, the Du’aa' mentioned above is among the supplications said morning and evening, so the merits of and reward for saying Du’aa' neither depends on a certain sitting position, nor is it limited to the end of the two Prayers.
Allah knows best.