Muwatta Malik, Book 18, Hadith 641 (In-Book) ⓘ

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.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَنِ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، وَعُثْمَانَ بْنَ عَفَّانَ، كَانَا يُصَلِّيَانِ الْمَغْرِبَ حِينَ يَنْظُرَانِ إِلَى اللَّيْلِ الأَسْوَدِ قَبْلَ أَنْ يُفْطِرَا ثُمَّ يُفْطِرَانِ بَعْدَ الصَّلاَةِ وَذَلِكَ فِي رَمَضَانَ ‏.‏