Muwatta Malik 553

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عَلِيَّ بْنَ أَبِي طَالِبٍ، كَانَ يَتَوَسَّدُ الْقُبُورَ وَيَضْطَجِعُ عَلَيْهَا ‏.‏ قَالَ مَالِكٌ وَإِنَّمَا نُهِيَ عَنِ الْقُعُودِ عَلَى الْقُبُورِ فِيمَا نُرَى لِلْمَذَاهِبِ ‏.‏

ahya related to me from Malik that he had heard that AIi ibn Abi Talib used to rest his head on graves and lie on them. Malik said, "As far as we can see ,it is only forbidden to sit on graves to relieve oneself

Reference

Muwatta Malik 553

In-book

Book 16, Hadith 34