Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: For the funeral prayer there is neither a fixed time nor a fixed number. He meant the takbir.
Al-Bazzar recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجنائز / 4175