Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) commanded the night prayer and encouraged it, to the extent that he said: "Hold fast to the night prayer, even if it is a single rak'ah." Then the Messenger of Allah (peace and blessings be upon him) came out and saw a man bowing (in prayer) after the prayer had been established, and he also said: "Will you not desist? Two prayers together?"
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11481