(4) Prayer
كتاب الصلاة
(3a) Chapter: Observing Prayer Early in the Period - Section 1
(2) باب تعجيل الصلوات
Mishkat al-Masabih 602 
He also reported God’s Messenger as saying, “If any of you performs a sajda of the afternoon prayer before sunset, he should complete his prayer; and if he is present at a sajda of the Morning Prayer before sunrise, he should complete his prayer.” Bukhari transmitted it.
وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «إِذَا أَدْرَكَ أَحَدُكُمْ سَجْدَةً مِنْ صَلَاةِ الْعَصْرِ قَبْلَ أَنْ تَغْرُبَ الشَّمْسُ فَلْيُتِمَّ صَلَاتَهُ وَإِذَا أَدْرَكَ سَجْدَةً مِنْ صَلَاةِ الصُّبْحِ قَبْلَ أَنْ تَطْلُعَ الشَّمْسُ فَلْيُتِمَّ صَلَاتَهُ» . رَوَاهُ البُخَارِيّ
 صَحِيحٌ  (الألباني)حكم :
Reference : Mishkat al-Masabih 602
In-book reference : Book 4, Hadith 37