It is narrated from Sayyiduna al-Muttalib bin Abi Wada'ah (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) saw a man praying while sitting, and said: "The reward of one who prays sitting is half the reward of one who prays standing." So the people took on the hardship and began praying standing.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17821