What did the Prophet Muhammad (saw) teach Abu Bakr As-Siddiq to be used as an invocation during prayer?
 Narrated Abu Bakr As-Siddiq:                     I asked Allah's Apostle to teach me an invocation so that I may invoke Allah with it in my prayer. He told me to say, "Allahumma inni zalumtu nafsi zulman kathiran, Wala yaghfiru dh-dhunuba illa anta, fa ghfir li maghfiratan min `indika, wa r-hamni, innaka anta l-ghafuru r-rahim (O Allah! I have done great injustice to myself and none except You forgives sins, so bestow on me a forgiveness from You, and Have Mercy on me, You are the Forgiver, the Merciful).