Hadith 17:587
Abu Mu'awiyah narrated to us, from Ibn 'Awn, from Ibn Sirin, who said: Ibn 'Umar had a need for Mu'awiyah, so he wanted to write to him. They said, "If only you would begin with him [i.e., by addressing him as 'Amir al-Mu'minin']." They did not cease [urging] him until he wrote: "In the name of Allah, the Most Gracious, the Most Merciful. To Mu'awiyah."
حدثنا أبو معاوية عن بن عون عن بن سيرين قال كانت لابن عمر حاجة إلى معاوية فأراد أن يكتب إليه فقالوا لو بدأت به فلم يزالوا به حتى كتب بسم الله الرحمن الرحيم إلى معاوية