We were with the Messenger of Allah (ﷺ) on a journey and he was observing Saum (fasting). When the sun set, he (ﷺ) said to a person, "Dismount and prepare the ground roasted barley drink for us." Upon this he replied, "O Messenger of Allah, there is daylight still." The Messenger of Allah (ﷺ) said, "Get down and prepare barley drink for us." He said, "But it is still daytime." The Messenger of Allah (ﷺ) again said to him, "Get down and prepare barely drink for us." So he got down and prepared a barley liquid meal for him. The Prophet (ﷺ) drank that and then said, "When you perceive the night approaching from that side, a person observing Saum (fasting) should break the fast." And he (ﷺ) pointed towards the east with his hand.

[Al-Bukhari and Muslim].