Musannaf Ibn Abi Shayba 3:5965

Abu Bakr narrated to us, he said: Waki’ narrated to us, from ‘Isa bin Hafs bin ‘Asim, from ‘Ata’ bin Abi Marwan al-Aslami, from his father, who said: We went out with ‘Umar bin al-Khattab to seek rain, and he did not increase [his prayer] beyond the seeking of rain.