Mukhtasar Bukhari

Narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When any one of you gets one rak'ah of the Asr prayer before the sun sets, he should complete his prayer, and when he gets one rak'ah of the Fajr prayer before the sun rises, he should (also) complete his prayer."

Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) heard from the Messenger of Allah (peace and blessings be upon him) that he (peace be upon him) used to say: "The duration of your existence, in comparison to the nations that passed before you, is like the time between the Asr prayer and sunset. The people of the Torah were given the Torah and they worked until half the day had passed; they became tired and were each given one qirat. Then the people of the Gospel were given the Gospel and they worked until the Asr prayer; then they became tired and were each given one qirat. After that, we were given the Qur'an and we worked until sunset, so we were given two qirats each. Both the people of the Book said, 'O our Lord! You gave these people two qirats and gave us only one qirat, even though we did more work.' Allah the Exalted said, 'Did I withhold anything from your wages?' They replied, 'No.' So Allah the Almighty said, 'This is My mercy; I give it to whom I will (what objection do you have?).'"