Yahya related to me from Malik from Ibn Shihab that Said ibn al-Musayyab said, "Which prayer is it when you remain in the sittingposition in every raka?'' Said said, "It is maghrib when you miss oneraka, and that is the sunna in all the prayers."
USC-MSA web (English) reference: Book 9, Hadith 83
Arabic reference: Book 9, Hadith 414