If anyone says a rak'ah of the 'Asr prayer before sunset, he has observed (the 'Asr prayer), and if anyone performs a rak'ah of the Fajr prayer, he has observed (the Fajr prayer). (Using translation from Abū Dāʾūd 412)
قَالَ رَسُولُ اللهِ ﷺ مَنْ أَدْرَكَ سَجْدَةً مِنَ الْعَصْرِ قَبْلَ أَنْ تَغْرُبَ الشَّمْسُ وَمِنَ الْفَجْرِ قَبْلَ أَنْ تَطْلُعَ الشَّمْسُ فَقَدْ أَدْرَكَهَا