Maliks Muwatta Book 18, Hadith Number 8.
Section : Being Quick to Break the Fast.
Yahya related to me from Malik from Ibn Shihab from Humayd ibn Abd ar-Rahman that Umar ibn al-Khattab and Uthman ibn Affan would pray maghrib when they saw the night darkening, before they broke their fast, and that was during Ramadan.