He said, "I will surely observe the prayer of the Messenger of Allah ﷺ tonight."He continued, "So I lay down using his threshold," or "his tent," "as a pillow. The Messenger of Allah ﷺ prayed two light rakahs, then he prayed two rakahs that were long, long, long, then he prayed two rakahs shorter than the two before them, then he prayed two rakahs shorter than the two before them, then he prayed two rakahs shorter than the two before them, then he prayed two rakahs shorter than the two before them, then he prayed witr. That made thirteen rakahs in all."
حدثنا القعنبي عن مالك عن عبد الله بن أبي بكر عن أبيه أن عبد الله بن قيس بن مخرمة أخبره
أنه قال لأرمقن صلاة رسول الله ﷺ الليلة قال فتوسدت عتبته أو فسطاطه فصلى رسول الله ﷺ ركعتين خفيفتين ثم صلى ركعتين طويلتين طويلتين طويلتين ثم صلى ركعتين وهما دون اللتين قبلهما ثم صلى ركعتين دون اللتين قبلهما ثم صلى ركعتين دون اللتين قبلهما ثم صلى ركعتين دون اللتين قبلهما ثم أوتر فذلك ثلاث عشرة ركعة
Muslim narrated it in the Book of the Prayer of Travelers, Chapter on the Prayer and Supplication of the Prophet ﷺ at Night, hadith 765, from the hadith of Malik through this route, and it is found in Muwatta (Yahya's version), 1/122.