Abdullah bin Muhammad bin Umar (may Allah have mercy on him) narrates that his father (may Allah have mercy on him) said: The Messenger of Allah (peace and blessings be upon him) sprinkled water on the grave of his son. And he says: I do not know otherwise than that he (also) threw soil on it with his hands.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6742