Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1361
Narated By Khalid al-Juhani : I shall watch the prayer of the Apostle of Allah (pbuh) at night. I slept at the threshold of his door or of his tent. The Apostle of Allah (pbuh) prayed two light rak'ahs, and then he prayed two long, long, long rak'ahs. He then prayed two rak'ahs that were not so long as the two rak'ahs before them; he then prayed two rak'ahs that were less in duration, then the rak'ahs before them; again he prayed two rak'ahs that were less in length than the preceding rak'ahs; he again prayed two rak'ahs that were less in length than the previous rak'ahs. This made altogether thirteen rak'ahs.