Abdullah bin Amir bin Rabi'ah narrates from his father (may Allah be pleased with him): I saw the Prophet (peace and blessings be upon him) when Uthman bin Maz'un (may Allah be pleased with him) was buried. He (peace and blessings be upon him) performed his funeral prayer and said four takbirs over him, and threw earth three times with both his hands while he (peace and blessings be upon him) was standing at the grave.
Its chain is weak, but it has a supporting report through Ja'far bin Muhammad from his father, from the Prophet (peace and blessings be upon him) in mursal form, and it is also narrated from Abu Hurayrah (may Allah be pleased with him) as marfu'. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6730