The phrase "Iyyaka na'budu wa iyyaka nasta'in" is found in Surat Al-Fatihah. Al-Fatihah is the opening chapter of the Quran, and it holds a special significance in Islamic prayer as it is recited in every unit of the Muslim prayer (Salah).
Al-Fatihah, also referred to as "The Opening," consists of seven verses. The phrase in question means "You alone we worship, and You alone we ask for help", emphasizing the monotheistic worship of and reliance on Allah.