"Indeed Allah wants to keep away the uncleanliness from you, O people of the House! and to purify you with a (thorough) purifying." Qur'an 33:33
Ma'sum (pl. Ma'sumeen) means the sinless or infallibles ones. When the Prophet (pbuh) recited the aya 33:33 he said: "Thus Me and my Ahlulbayt are clear from sins."
"Whoever obeys the Messenger, he indeed obeys Allah" 4:80
"...and obey not from among them a sinner or an ungrateful one." 76:24
The ayat conclude that obedience is only of those who are sinless (Ma'sum). Ahlulbayt literally means people of the house referring to one's family. In Islam the term refers to the family of the Prophet (pbuh) for whom he said: "I leave among you two weighty things, that if you remain attached to those two you will not go astray after me. They are the Book of Allah and my progeny, my Ahlulbayt."