Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) stayed with me for a month, and I saw that when the sun declined - or passed the meridian, or as he said - if he was engaged in some worldly task he would leave it, and if he was asleep it was as though he had been woken up. Then he would get up and take a ritual bath (ghusl) or perform ablution (wudu), then pray four rak'ahs, which he would complete, perform excellently and be at ease in. When he (peace and blessings be upon him) intended to leave (my house), I said: "O Messenger of Allah, I see that when the sun declines or passes the meridian, if you have some worldly task in hand you leave it, or if you are asleep it is as though you are woken up; then you take a ritual bath or perform ablution, then pray four rak'ahs which you complete, are at ease in and perform excellently?" The Messenger of Allah (peace and blessings be upon him) said: "At that hour the gates of the sky - or the gates of Paradise - are opened, and at that time no one is performing this prayer, so I love that some good deed should ascend from me to my Lord at that hour."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3839