مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3613
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdur-Rahman bin Yazeed said: 'Abdullah (رضي الله عنه) said: I never saw the Messenger of Allah (ﷺ) offer any prayer that was not on time except in two cases: Maghrib and 'Isha' in Muzdalifah, and he prayed Fajr on that day before the time when he usually prayed it (but after ascertaining that dawn had broken).
Musnad Ahmad 3637lts isnad is saheah, al-Bukhari (1682) and Muslim (1289)]