Musannaf Ibn Abi Shayba 17:1215

Ibn Idris, Ibn ‘Uyaynah, and Abu Khalid al-Ahmar narrated to us, from Muhammad ibn ‘Ajlan, from Bukayr ibn ‘Abd Allah ibn al-Ashajj, from Ma’mar, from ‘Ubayd Allah ibn ‘Adi ibn al-Khiyar, who said: ‘Umar said: "Do not make Allah hateful to His servants. One of you may be an Imam, so he makes what he is in (the prayer) long for them. And one of you may be a storyteller (qass), and he makes what he is in long for them."