Standing in the prayer is a must if you have the abilty as Allaah mentions in Surah Baqarah v. 238 Stand up for Allah…..
If one is able to stand up, and he didn’t his Salah is invalid. The proof is the hadith of Imran bin Husayn: Pray (command) standing up, if you’re not able, pray sitting….
As for the voluntary prayers: you can pray sitting but will get only half the reward. Better to pray standing up. (See the hadith of Amr bin al ‘Aas in Saheeh Muslim)
[Taken from the lessons on “Conditions, Pillars and Obligation of Salah – Shaykh ul Islaam Muhammad bin Abdil Wahhab رحمه الله Explanation by Shaykh Abdul Muhsin al abbaad حفظه الله Taught by: Abdulilah Lahmami]