Musannaf Ibn Abi Shaybah

Hazrat Ibn Abbas (may Allah be pleased with them both) narrates that the Noble Prophet (peace and blessings be upon him) stayed and prayed for seventeen days and he used to shorten the prayer. Hazrat Ibn Abbas (may Allah be pleased with them both) says that whoever stays and prays for seventeen days should shorten the prayer, and whoever intends to stays longer and prays than that should offer the full prayer.