Abu Awanah said: I saw Hasan al-Basri (may Allah have mercy on him) on the day of Arafah after Asr; he supplicated to Allah and made remembrance, and the people gathered around him.
And in the narration of Muslim (bin Ibrahim) it says: I saw al-Hasan come out of the maqsurah (the screened-off enclosure) on the day of Arafah after Asr; then he sat down and observed (the gathering of) Arafah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9476
Shu'bah (may Allah have mercy on him) said: I asked al-Hakam and Hammad about people gathering in the mosques on the day of Arafah, and both of them said: It is an innovation (bid'ah).
It is narrated from Ibrahim (may Allah have mercy on him) that it is an innovation, and it is reported from Qatadah (may Allah have mercy on him) through al-Hasan (may Allah have mercy on him) that the first to do this was Ibn Abbas (may Allah be pleased with them both).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9477