Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "If any of you performs the first rak'ah of the 'Asr prayer before the sun sets, then he should complete his prayer (by continuing it). (Similarly,) whoever performs the first rak'ah of the Fajr prayer before the sun rises, then he should also complete his prayer (by continuing it)." (That is, in these situations the prayer will be considered performed, not made up as qada.)