Ja'far bin Muhammad narrates from his father that the Messenger of Allah (peace and blessings be upon him) sprinkled water on the grave of (his son) Ibrahim and placed pebbles on it. Al-Shafi'i (may Allah have mercy on him) says: Pebbles stay only on a level grave.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6740