Rabbi Elazar/Eliezer ben Rabbi Tzaddok citing a practice of the household of Rabban Gamliel: Beitzah 14b
Rabbi Elazar/Eliezer ben Rabbi Tzaddok's question to the Sages in Usha, and his own re-telling of what was said in Yavneh: Niddah 14b-15a
Rabbi Elazar/Eliezer ben Rabbi Tzaddok claimed his lineage was from Sinav ben Binyamin, a family which brought wood for the Temple oferings for the tenth of Av in the time of Ezra, so that the tenth of Av was a holiday for them: Eruvin 41a