Sayyiduna Ibn Mas'ud (may Allah be pleased with him) said, concerning the saying of Allah Almighty: "Those who remember Allah standing, sitting and (lying) on their sides" [Surah Al Imran: 191]: "If you cannot pray standing, then pray sitting, and if you cannot do that either, then pray lying down."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9002