Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: Once I was discussing voluntary prayers at night, and someone said that the Prophet (peace and blessings be upon him) said: "Half of it (the night), or a third of it, or a quarter of it, or the time it takes to milk a she-camel, or the time it takes to milk a goat (one should spend in voluntary prayer)."
Abu Ya'la recorded it, and its narrators are the narrators of the Sahih.