Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 910
Abu Sa'id al-Khudri reported: We used to estimate how long Allah's Messenger (may peace be upon him) stood in the noon and afternoon prayers, and we estimated hat he stood in the first two rak'ahs of the noon prayer as long as it takes to recite Alif Lam Mim, Tanzil, i. e. as-Sajda. We estimated that he stood half that time in the last two rak'ahs; that he stood in the first two of the afternoon as long as he did in the last two at noon; and in the last two of the afternoon prayer about half that time.
Abu Bakr in his narration has made no mention of Alif Lam Mim, Tanzil, but said: As long as it takes to recite thirty verses.