Majma al-Zawaid — Hadith 3318

Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: When it was midday, the Prophet (peace and blessings be upon him) would go to one of the gardens of Madinah, where water for purification had been prepared for him. If he needed to relieve himself, he would do so; otherwise he would perform ablution. When the sun had declined from the middle of the sky by the length of a sandal strap, he would get up and pray four rak'ahs. He did not recite the tashahhud between them, and he gave the salam after the fourth rak'ah. Then he would get up and come to the mosque. Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: "O Messenger of Allah! What is this prayer that you perform and we do not perform?" The Prophet (peace and blessings be upon him) said: "Whoever of my Ummah performs this prayer, it is as though he has spent that night in worship. It is an hour in which the gates of heaven are opened and in which supplication is answered."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains Nafi' Abu Hurmuz, who is matruk (abandoned).