Musannaf Ibn Abi Shayba 3:5257

Hafs ibn Ghiyath narrated to us, from Asim, from Abu Uthman, who said: A man bought a garden in Medina and profited from it one hundred full-grown palm trees. The Prophet, peace and blessings of Allah be upon him, said: "Shall I not inform you of something better than this? A man who performed ablution and perfected the ablution, then prayed two rak'ahs in a cave or at the foot of a mountain—that is a better profit than this."