Maliks Muwatta Book 9, Hadith Number 83.
Section : How to Do the Prayer in General.
Yahya related to me from Malik from Ibn Shihab that Said ibn al-Musayyab said, “Which prayer is it when you remain in the sitting position in every raka?” Said said, “It is maghrib when you miss one raka, and that is the sunna in all the prayers.”