Musannaf Ibn Abi Shayba 3:2991

Hushaym narrated to us, he said: 'Ata' ibn al-Sa'ib informed us, from Abu 'Abd al-Rahman, who said: Ibn Mas'ud came to us, and he used to command us to pray four [rak'ahs] after Jumu'ah. Then when 'Ali came to us, he commanded us to pray six. So we took the statement of 'Ali and left the statement of 'Abd Allah. He said: We used to pray two [rak'ahs], then four.