Sayyiduna Zayd bin Khalid al-Juhani (may Allah be pleased with him) relates: I kept watching the prayer of the Prophet (peace and blessings be upon him) throughout the night. I was leaning against the tent. The Prophet (peace and blessings be upon him) prayed two light rak'ahs. Then he (peace and blessings be upon him) prayed two long rak'ahs, then he prayed two rak'ahs lighter than the first ones, then after that two rak'ahs lighter than those. Then he (peace and blessings be upon him) prayed two rak'ahs that were lighter still, then he (peace and blessings be upon him) prayed two rak'ahs lighter than those, then he (peace and blessings be upon him) prayed witr. These were thirteen rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4683