How many times is the supplication for forgiveness (Istighfar) mentioned to have been recited by the Prophet, peace and blessings be upon him, after the Fajr and Maghrib prayers?

The Prophet, peace and blessings be upon him, used to seek Allah's forgiveness three times after prayer and say: "O Allah, You are Peace, and from You comes peace. Blessed are You, O Possessor of Majesty and Honor." And he would say: "O Allah, help me to remember You, to thank You, and to worship You in the best manner." He also advised Mu'adh to recite this supplication after every prayer.