Musnad Ahmad 962:16

Hammad ibn Khalid narrated to us, Mu'awiyah—meaning Ibn Salih—narrated to us, from al-Safar ibn Nusayr, from Yazid ibn Shurayh, from Abu Umamah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "None of you should come to the prayer while he is suppressing the urge to urinate, and he should not enter a house except with permission, and an Imam should not lead a people in prayer and then single himself out with a supplication to the exclusion of them."