Is the Muhammadan prayer: "O Allah, bless our Master Muhammad son of Abdullah, who upholds the truth of Allah, who said: 'No matter is straitened but Allah eases it,'" authentic?

Praying upon the Prophet is among the best acts of drawing near to Allah, and the best of its forms is the Ibrahimiyyah prayer (Salat al-Ibrahimiyyah). The form mentioned in the question is not authentically transmitted, and it contains a saying attributed to the Prophet, peace and blessings be upon him, which he did not utter in that exact wording. Therefore, it should be avoided.