Sayyiduna Amir bin Rabi'ah (may Allah be pleased with him) says: I saw the Prophet (peace and blessings be upon him) leading the funeral prayer of Sayyiduna Uthman bin Maz'un (may Allah be pleased with him); he said four takbirs, stood at his grave, and threw three double handfuls (of earth) onto him.
Al-Tabarani recorded it in al-Mu'jam al-Kabir. Its chain contains al-Qasim bin Abdullah al-Umari, who is matruk (abandoned).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجنائز / 4184