Sunan an-Nasa'i, Volume 3, Book 24, Hadith 2874 (DarusSalam) ⓘ
It was narrated that Ibn Abbas said: "The Messenger of Allah came on the fourth day of Dhul-Hijjah having entered Ihram for Hajj. He prayed Subh in Al-Batha and said: 'Whoever wants to make it Umrah, let him do so.'"
أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، عَنْ يَحْيَى بْنِ كَثِيرٍ أَبُو غَسَّانَ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ أَيُّوبَ، عَنْ أَبِي الْعَالِيَةِ الْبَرَّاءِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قَدِمَ رَسُولُ اللَّهِ صلى الله عليه وسلم لأَرْبَعٍ مَضَيْنَ مِنْ ذِي الْحِجَّةِ وَقَدْ أَهَلَّ بِالْحَجِّ فَصَلَّى الصُّبْحَ بِالْبَطْحَاءِ وَقَالَ " مَنْ شَاءَ أَنْ يَجْعَلَهَا عُمْرَةً فَلْيَفْعَلْ " .
also referenced under: Hadith 2871 (Reference) & Book 24, Hadith 0 (In-Book)