Musnad Ahmad 834:21

Muhammad ibn Ja'far narrated to us, Kahmas narrated to us, Ibn Buraydah narrated to me, he said: Ibn Mughaffal said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Between every two calls [to prayer] is a prayer, between every two calls [to prayer] is a prayer, between every two calls [to prayer] is a prayer for whoever wishes."