Sahih al-Bukhari 3233
Sahih al-Bukhari · Book 59: Beginning of Creation · If anyone says Amin [during the Salat (prayer) at the end of the recitation of Surat Al-Fatiha]
Narrated `Abdullah:
حَدَّثَنَا حَفْصُ بْنُ عُمَرَ ، حَدَّثَنَا شُعْبَةُ ، عَنِ الأَعْمَشِ ، عَنْ إِبْرَاهِيمَ ، عَنْ عَلْقَمَةَ ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه – {لَقَدْ رَأَى مِنْ آيَاتِ رَبِّهِ الْكُبْرَى} قَالَ رَأَى رَفْرَفًا أَخْضَرَ سَدَّ أُفُقَ السَّمَاءِ.
Regarding the Verse: "Indeed he (Muhammad) did see. Of the Signs of his Lord, The Greatest!" (53.18) That the Prophet (ﷺ) had seen a green carpet spread all over the horizon of the sky.