Musnad Ahmad 1018:36

Yazid narrated to us, Mas'udi informed us, from Zayd al-'Ammi, from Abu Nadra, [from Abu Sa'id]. Yazid said: Sufyan informed us, from Zayd al-'Ammi, from Abu al-'Aliyah, who said: Thirty of the companions of the Prophet, may Allah bless him and grant him peace, gathered and said: "As for what the Messenger of Allah, may Allah bless him and grant him peace, recites aloud, we have learned it. And as for what he does not recite aloud, we do not measure it by what he recites aloud." He said: So they gathered, and no two of them differed that the Messenger of Allah, may Allah bless him and grant him peace, used to recite in the Dhuhr prayer the amount of thirty verses in the first two rak'ahs, in each rak'ah, and in the last two rak'ahs the amount of half of that. And he used to recite in the 'Asr prayer in the first two [rak'ahs] the amount of half of his recitation in the first two rak'ahs of Dhuhr, and in the last two [rak'ahs] the amount of half of that.