It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that he - and he was a man of ease - asked the Messenger of Allah (peace and blessings be upon him) about a man praying standing and sitting, and he (peace and blessings be upon him) said: "His prayer sitting is equal to half of his prayer standing."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15680