Musannaf Ibn Abi Shayba 3:2299

Waki’ and Ibn Mahdi narrated to us, from Sufyan, from al-Nu’man ibn Qays, who said: When the women would pass by ‘Ubaydah while he was praying, they would say, "Lighten [the prayer], for it is the prayer of ‘Ubaydah," meaning because of its lightness.