Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) covered the grave of Sa'd with a cloth.
I know it only from the hadith of Yahya bin Uqbah bin Abi al-Ayzar, and he is da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 7049