'Aasim bin Damrah (Allah have mercy on him) said: "We asked ‘Alee about the voluntary (prayer) of Allah’s Messenger (sallallaahu alayhi wa sallam) during the day. Then He said: ‘You will not be able to do it.’
Fiqh of the Hadeeth: Shaykh al-Albaanee (may Allah have mercy on him) said, "His saying, 'He would say the tasleem at the end' indicates that the Sunnah with regard to four rak'ah voluntary prayer during the day
Source: Nudhum al-Faraa’id mimma fee Silsilati al-Albaanee min Fawaa’id, vol. 1, p. 400-401 by Abdul-Lateef bin Muhammad bin Ahmad bin Abee Rabee’.