Sunan Abi Dawud 1366

Khalid al-Juhani said:I shall watch the prayer of the Messenger of Allah (ﷺ) at night. I slept at the threshold of his door or of his tent. The Messenger of Allah (ﷺ) prayed two light rak'ahs, and then 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, than the rak'ahs before them; again he prayed two rak'ahs that were less in length then the preceding rak'ahs; he then prayed two rak'ahs that were less in length than the previous rak'ahs. This made altogether thirteen rak'ahs