Sayyiduna Abu Umamah (may Allah be pleased with him) narrates regarding (the words of Allah Almighty): "as an additional (voluntary) act for you." Sayyiduna Abu Umamah (may Allah be pleased with him) says: This was a voluntary act specifically for the Prophet (peace and blessings be upon him).
Ahmad recorded it, and al-Tabarani recorded something similar in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat. In his narration he recorded these words: "And during the night, offer Tahajjud; it will be an additional (voluntary) act for you." In al-Mu'jam al-Kabir he recorded these words: "This prayer was voluntary for the Prophet (peace and blessings be upon him), and for you it is a merit." Some of the chains of Ahmad and some of the chains of the others are hasan (good).