Humayd bin Abd al-Rahman (may Allah have mercy on him) said that Umar and Uthman (may Allah be pleased with them both) used to perform the Maghrib prayer when they saw that the night had become dark. Then they would break their fast after the prayer, and this was in Ramadan.
Imam al-Shafi'i (may Allah have mercy on him) said in al-Mabsut that they did this seeing latitude in delaying it, not that their aim was to abandon the virtue that was permitted to them; and they would become non-fasting even without eating or drinking, because the fast does not remain valid at night.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8126