Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"Thirty of the Companions of the Messenger of Allah (saw) who had been at Badr came together and said: 'Come, let us estimate the length of the recitation of the Messenger of Allah (saw) for the prayer in which Quran is not recited out aloud.' No two men among them disagreed, and they estimated the length of his recitation in the first Rakah of the Zuhr to be thirty Verses and in the second Rakah to be half of that. They estimated his recitation in Asr to be half of the last two Rakah of Zuhr." (Daif)