Mishkat al-Masabih, hadith 1626

وَعَنِ الْقَاسِمِ بْنِ مُحَمَّدٍ قَالَ: دَخَلْتُ عَلَى عَائِشَةَ فَقُلْتُ: يَا أُمَّاهُ اكْشِفِي لِي عَنْ قَبْرِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَصَاحِبَيْهِ فَكَشَفَتْ لِي عَنْ ثَلَاثَةِ قُبُورٍ لَا مُشْرِفَةٍ وَلَا لَا طئة مَبْطُوحَةٍ بِبَطْحَاءِ الْعَرْصَةِ الْحَمْرَاءِ. رَوَاهُ أَبُو دَاوُدَ

Al-Qasim b. Muhammad told of his going to visit ‘A’isha and saying, “Mother, show me the grave of the Prophet and his two companions.” She showed him three graves which were neither high nor low, but were spread over with soft red pebbles in an open space. Abu Dawud transmitted it.