Al-Kāfi - Volume 3 > Preparing Lahad and the Materials Needed (Ch. 64 is best explained by fatwa.)
Hadith #1

1ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنْ عَلِيِّ بْنِ مُحَمَّدٍ الْقَاسَانِيِّ قَالَ كَتَبَ عَلِيُّ بْنُ بِلاَلٍ إِلَى أَبِي الْحَسَنِ (عَلَيْهِ الْسَّلام) أَنَّهُ رُبَّمَا مَاتَ الْمَيِّتُ عِنْدَنَا وَتَكُونُ الأرْضُ نَدِيَّةً فَنَفْرُشُ الْقَبْرَ بِالسَّاجِ أَوْ نُطْبِقُ عَلَيْهِ فَهَلْ يَجُوزُ ذَلِكَ فَكَتَبَ ذَلِكَ جَائِزٌ.

1. Ali ibn Ibrahim has narrated from his father from Ali ibn Muhammad al-Qasaniy who has said the following: “Once Ali ibn Bilal wrote to abu al-Hassan, ‘Alayhi al-Salam, about the condition of a grave asking, ‘Someone may die in our area but the ground of the grave is wet.’ We pave it (the floor) with teak (wooden board) or cover it in this way, is this permissible? He (the Imam) wrote the answer, ‘It is permissible.’” H 4536, Ch. 64, h 2