Imam Al-Nawawi’s Riyad-us-Saliheen
The Excellence of Performing two Rak’ah of Voluntary Prayer after Ablution
1146. Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (sallallaahu alayhi wa sallam) said to Bilal (May Allah be pleased with him) “Tell me about the most hopeful act (i.e., one which you deem the most rewarding with Allah) you have done since your acceptance of Islam because I heard the sound of the steps of your shoes in front of me in Jannah.” Bilal said: “I do not consider any act more hopeful than that whenever I make Wudu’ (or took a bath) in an hour of night or day, I would immediately perform Salat (prayer) for as long as was destined for me to perform.”
[Al-Bukhari and Muslim].